Arboreal species for a 4x2x2 enclosure

[WrongPython]

Oooo, did I miss something? Are you getting new cages Ziggy?

I largely agree with what the others have said so far. A 4'x2'x2' should be minimally sufficient for all but the larger Morelia (ie. the larger coastals, bredli, and diamonds). Something along the lines of a Papuan, inland, or male jungle, coastal, or Darwin would probably find a cage that size comfortable. A smaller female of the later three would be fine as well. It could potentially work for a male bredli as well if you watch how much you feed them, but there's still a good chance they'd outgrow it over the years. An island locality boa would also be an excellent choice if you'd like something a little different -- most of the island boas are more arboreal, so they'd probably appreciate any perching offered and make good display boas.

There's definitely nothing wrong with building bigger cages for the above snakes, either. My personal long term goal is to build 6' display cages for all of my snakes even though they don't strictly need cages that large. They've always used every extra inch of cage space I've given them, and it's nice seeing snakes stretched out, active, and comfortable. If you kit a big cage out properly, most snakes will greatly appreciate the extra space.