I like the solidness of the 4x4s so yes I would use them again. On a 4' tall tank I would think a 4x4 is the way to go for sure as the pressure difference between a 3' tall tank and a 4' tall tank is a bit more and I would also start to look at all the support structures on the bottom portion of the tank (supports, plywood, fiberglass, bottom window brace, etc.) with that in mind I would use a 4x4 for sure on a 4' tall tank.
A 4' tall tank is out of my comfort zone so all this is strictly opinion based on my limited experience so there is a good chance I am wrong.
Love watching big builds so please keep us posted when you get going.