Let's start with "buying the right pony"
The "right pony" for a 4 year old is one that is safe, sound, sane, competent, well trained, friendly, and wants to please.
While such ponies are undeniably "worth their weight in gold", they are often not very expensive. You just have to look.
The law of supply and demand is such that ponies like that which are ALSO capable of winning in the "A" show ring are VERY expensive.
But, IMNSHO, winning in the show ring is completely irrelevant in being the "right pony" for a 4 year old.
Show fees don't have to be expensive, unless you insist on going to the expnesive shows. And once you are jumping 3'6", or higher there may be good reasons to go to the expensive shows. But there are plenty of good inexpensive shows.
This weekend I went to a VHSA, but not USEF recognized, jumper show. I did 3 classes between 3' and 3'6". Jumps were good. Courses were well designed.
I paid $61. Entry fees were $79, but I "won back" $18 by finishing 3rd, 4th, and 5th.
It was 50 miles each way, I get 10 miles to the gallon towing my trailer, and gas was $3.25. So I spent about $32.50 on trailering.
Yes, I also spend a couple hundred dollars on entries to go to Warrenton, Upperville, or HITS. And I get to test myself against a larger number of riders. But there are some pretty talented riders at the other shows too.