First off, welcome to sSNAKESs!!
For a juvenile, that would be okay, but an adult? Then the 12" depth seems awfully shallow, even with the 4' length and the 21" height. You're also calling it a "tank" so I guessing that it is glass sided? Then possibly either a UTH with a basking lamp, a UTH w/CHE, or a CHE/basking lamp combination. I'm using a wooden enclosure for my CP, so it has an RHP to keep the temps up.
For an adult CP, like my 8.6' CP, it seems a little cramped in his 4'x'2'16" (l-w-h) enclosure, and I'm thinking of a 6'x3'x30", or an 8'x3'x30", for a bit more room to "roam" around in. If I could cough up the $1k for a 4x2x6' arboreal I'd go for that, but that won't happen until maybe Nov or Dec time frame.
I've had mixed experiences moving from Live to F/T with some of my snakes. I tried converting my Dumerils to F/T but it refuses to eat anything but live pinky rats, even after starving him for a few weeks. Then there's my two 8yr old Anthill Pythons are live feeders as well, but the previous owner said they tried to convert them over to F/T for years but were unsuccessful as well. And now, I have a juvenile Zebra JCP that is only eating live adult mice, which presents two problems--getting it to start feeding on rats and also on F/T rats. Fun times ahead. Hopefully someone else has some information for you that I can use as well.