I know on a few camaros ive seen the exhaust pipe crushed into an oval, so the internal volume is still 4" but it will be like 2" x 5.5"
not sure how the layout is of your exhaust, but that could be an option?
also your going to go FROM a 4" to a
larger downpipe? wtf engine swap did you do, a diesel?
if you intentionally are wanting to go from 4" to dual pipes....
A (area) = π(pie)r(radius)2(squared)
A = 12.566370614359 in2 (at 4")
A = 7.068583470577 in2 (at 3")
A = 4.9087385212341 in2 (at 2.5")
A = 3.1415926535898 in2 (at 2")
so to go from a 4" and have equal volume, you would need 2-3" pipes, 3-2.5" pipes, or 4-2" pipes.