Well for the Full Grown Carpet I'd recommend about a 7'long x 3'wide x 3'high or maybe just to keep it square you could build the same enclosure i built for my bci which is a 8'L x 4'W x 4'H. As Costals usually max around the same size as a bci. Your 4'Wx3'Dx4'H would be better suited for say a Jungle or IJ Carpet. I'd stick with the overhead heat hence its a Semi-Arboreal Snake and so it doesn't mind the climb and being perched for basking purposes plus then that leaves the ground as a more cool/comfortable area.