Have you talked to the dealer about using the Red Hawk for Multiples? Looking it up at Running Brook, the 350 pound weight is rather light. Solid tire wheels look rather airy, not substantial in spoke size for weight of a Four vehicle. It doesn't SAY these are made for Multiples, Pairs OR Fours.
Using a single animal vehicle for Mulitiples may be "exciting", because it could get damaged. I am pretty sure the pony sized Eagle is not strong enough for a Four, even smaller animals. I have seen Eagles with pony Pairs, looked OK. But with Four of anything, that is a LOT of power pulling on the parts.
Lightweight vehicles in the past, have not proven to be durable or strong, when used with the wrong sized animals, even if only a Single! Just too much strength in the animal, it breaks or damages the connections or pieces/parts in situations where animals exert their power. I am referring to a period of time where the Drivers used the smallest possible vehicle in Marathon on CDEs, so horse had less work to move his load. There were MANY issues and problems that happened, with all kinds of well made vehicles put on horses too big.
This site shows a number of Marathon type vehicles, says if they are singles, Pairs or pony Fours. You can click up the various models, get specific details on standard features, options you can pay extra for. Comparing single Pony and the Pairs or Fours vehicles, should show you a substantial difference in build with weight of metal used. Not promoting these vehicles or seller, just trying to show a visible reason why you DO NOT want to buy a single pony vehicle for use behind a Four. I am using Marathon styles, because it works so well for daily use, in fun driving or training with all the desirable features.
Have you TALKED to any carriage dealers about vehicles suitable for a Pony Four? It is really difficult to cover all the details by writing them all out here. You probably will want to have your "shopping list" written out to not forget the must-have items, so you cover each desired feature in a discussion. Heavier vehicles in our experience, are just made with stronger materials in work, than lighter built vehicles used to save weight. It is not just "dead weight" you are hauling when using such vehicles, but usually means strength of parts in difficult situations.
I emailed about the Eagle and then spoke to them directly on the phone and they didn't seem to have any issues with the idea of 4 ponies put to either vehicle...
Pole for pair standard $450.00
Pole Head - crab for 4 in hand with safety strap $65.00
Leader bars $225
The only Bird in Hand vehicle that mentions 4 in hand is the Wagonette. They offer a large and lightweight wagonette. The large says that it fits from 13HH up and the lightweight is from 12hh up. After some thought, I would much prefer a wagonette than a marathon vehicle. I don't have any plans for CDEs and with this I could do "events" still and some minor offroading.
Pole setup on the Wagonette appears to be a "fixed pole" which is self-supported, no pole weight on the wheel animals. Fixed poles work pretty well, might only be an issue with steep hills, so pole end pulls up or down on the animals.
This could be an issue in use, or not, depending on where you have to drive. A Marathon vehicle would have a spring-loaded pole, which has flex in springs to allow it more movement in steep up or down places you drive. Like going down thru a ditch and up out the other side.
Wagonette has a nice look to it. Does show it with the air filled tires, but probably available with hard rubber wheels. What is the weight?
I called and asked a few questions. It's approximately 875lbs but would be lighter in with the pony adaptions. They do make a spring pole for it and the carriage size is mostly the same no matter what size horse it's for except they do tweek a few things he said and the wheels are different sizes. Here's a photo I found of it with hard rubber tires and a spring pole.
Looking harder at this wagonette, (and I am more awake tonight!), it does show what appears to be a sprung pole, rather than a fixed pole that I thought it had. I didn't look under the pole socket well last night. Sorry.
I like that this vehicle has brakes on front and back. We actually use the front brakes much more often than the rear brakes on our vehicles. This one is longer bodied, so probably heavier too, than the wagonette you pictured with natural wood panels. BUT you do have all wheel braking, over just rear brakes. We have found that using the rear brakes CAN skew the rear-end of the carriage, so it is trying to jack-knife on you. Using front brakes first or as the only braking touch, you don't get the rear trying to pass the front! Does take some practice, to get used to this kind of braking.
The wooden wheels, flat rubber are nice looking, of a good size for that vehicle. Both vehicles come with a wedge seat for putting driver in a good body position. I would suggest a plastic or naugahyde type upholstery, because it is easy to keep looking nice, doesn't stain. Easy to get replaced if you need to. Fabrics will fade in sunshine, do stain sometimes. Can also absorb smells or dampness when stored.
As a personal point, I would somehow get the spring sytem under the pole padded and maybe covered, to protect the animals from getting cut or hurt on it if a situation should develop. System just has a number of sharp edges, bolt ends, places that a leg could get into, if animal should have an issue. You could probably do the padding yourself, then have a cover (boot?) made that is dark naugahyde material to zip up or velcro in place over all those parts. Would be fairly invisible with animals hitched, down there under the vehicle. Might be an option from the seller if you ask.
Getting the crab for pole is probably another option to order, if the pole head is shiny silver metal, usually stainless steel. Parts will match well. Don't get any kind of brass pole head, brass is a weak metal, breaks under stress. Painted pole head and crab is also quite acceptable to use on these vehicles. We have a black steel pole head and crab on one of our carriages.
I think when I called he said that the standard size was the 6 person, but it does come in a four and an 8.
Can you tell me a little about splinter bar, singletrees, with and without evener. And then yoke, spreader and chain (like draft hitches), and pole straps.. and which combination would be the more appropriate for me? MY guess would be no evener, with singletrees and straps?
Splinter bar by itself (traditional) has roller bolts, no singletrees. You need to have collars on the horses to use roller bolts, since there is no movement in the splinter bar itself. Modern vehicles often have put singletrees under the splinter bar so they can use breast collar harness. You need to drive your horses so BOTH are working, using this system. You won't get a see-saw effect in starting like an evener can do, if they don't step off evenly, which is good for training new animals. Singletrees are removable if not being used. Splinter bars, roller bolts, require traces with a loop to go around the roller bolt. You can order special traces with loops or make up slot traces into loops with a Monkey Puzzle sqare.
An evener is nice, to keep both animals working together, pulling equally. Again, Driver needs to look down, make sure animals are pulling equally, because one will ALWAYS be a bit lazy in a Pair. You can tie evener bar down with a new animal, get the same effect as singletrees pulling from under the splinter bar. Lets new one get used to moving off smoothly without being pulled on, moving with his partner on command. Then you can remove the tie, let the evener work as designed.
A vehicle like you pictured, wouldn't have a yoke in most cases, for use with a Four. You could probably order a pole with a yoke for a Pair, 2nd pole with a pole head (using pole straps) and a crab, for use with the Four. The yoke for Pair with a shorter modern pole, would be the bolted on the metal pole end. Yokes used with modern vehicles, have snap shackles for the breast collar chest rings. Yokes come in various widths, from chest center to chest center of both horses, to shorter widths that snap on offset chest rings of breast collars. Animals don't need the longer pole for control, using yokes fastened to the breast collars.
Yoke is not usually seen on the Wheelers of a Four, they have pole straps instead, run from chest of animal to the arms of the pole head, with the Leader Bars hung on the Crab out ahead of their chests. This pole strap use is with breast collars or full neck collars. A traditional pole is LONG, ending at the relaxed nose of Wheelers, so they can control the vehicle. Modern vehicle with sprung poles can offer various Wheeler fastening options, but too short a pole with a pole head, does add difficulty in vehicle control. Can make the Wheelers go crooked by pulling them inward and not giving them good leverage.
Guess you need to ask what other options are available for the pole, to hitch Wheelers chests to the pole on a Four. I see a number of home designed features, interesting concepts, but I won't recommend something you might not be able to get from the seller.
I am not familiar with the "spreader" term, so can't comment. I know about spreader rings for running reins, is that it? Carriage driving doesn't use spreader rings. Also not sure of your use of "chain", though on Coaches, they might use pole chains run from chest ring of full neck collar, to pole head, on the Wheelers. Chains used this way are not correct on certain kinds of vehicles, and are always used with a full neck collar harness only, in Carriage Driving. Not up on my Draft harness details for showing or daily use.
I can see a wagonette, modern made, with both the splinter bar and singletrees or the evener with singletrees. Both could look nice, be appropriate and easy on the animals.
The Mischka Driving Horse Calendar has a couple of Pairs with modern vehicles, one with a wide yoke, which you can't see much detail of. Yoke ends flat-even with the horse chests, snapped to breast collar harness, pole itself is short. The other shows the Pair with a short armed pole head, using straps to horse chest instead of snaps. Both give you an idea of how things are attached with Pairs, using different pole ends.
The lovely Four, shows the angle and length of Wheeler pole strap going ahead of his chest to the pole head. Horse is not using a short pole. Length allows Wheelers control of vehicle, plus gets the Leaders and swinging Leader Bars out far enough to NOT hit the Wheeler faces when pole or carriage bounces. Leader Bars can get quite lively on rough ground, because they are only weighted down with the traces. Leaders are almost never in draft to pull, not kept in place with pulling, so the Bars are free to move about and do!
Click Detailed Pictures, if the enlarged photos of the months don't just appear in scrolling down. That button is below the Calendar with the days shown. Button is in a row with several other choices.