Jealous of that four foot tank! Recently ordered a forty-long with slide out top for a baby sonoran gopher. Of course, it didn't arrive as scheduled... now, the store says next week...
(Tiny baby is in an old ten gallon, which suits her fine for now.)
Whenever I come across a cheap decent tank used, I snatch it up. Good for fish, good for terrariums, good for viv's.
Agree with Sully, if your snake isn't a hatchling, I'd just let it have the extra room.
Otherwise, maybe you need some fish!