You can get a converter plug for the flat 4 to go into a 6 round. I had to do that when I but electric brakes to haul my second trailer, a KB, but still wanted to use the Brenderup. When I sold the Brenderup (sadly but I need a 3 horse trailer now), I sold the plug with it as the buyer had a similar electric hook up.
Many Euro trailers are "flat four" since that's all you need given the trailer has inertial brakes. But if a 6 or 7 pin is needed to mate with the tow vehicle, the connector can be replaced or an adapter used.
My 2003 Brenderup Solo came with a 6 pin round receptacle standard. The trailer hitch electrical receptacle I had installed on my Volvo is a 6 pin round. The fit is snug and tight between the two ends. It has never failed to make a connection.
My Brenderup (2011 Baron) is a 6 pin. I did have to pull that off and wire in a big honking adapter for my Dodge Ram truck. I used the adapter for awhile but finally just wired in the mating plug for the truck.
I have the 7-round RV style on mine which is fine because that's what came factory with my truck. I always figured they would come from Brenderup with a 4-flat because they don't need the electric brake connector.