There are many varied opinions on this. Most breeders keep adult BPs in tubs their entire life. I had my breeder bps in 41 qt tubs (35"x16"). Some go even smaller. To me a 4x2x1 enclosure is too big for an adult BP and totally unnecessary, as that's the size I keep my adult boas in effectively. If you want to create a realistic viv with hides, plants and other stuff then I would say you can go bigger. If its just going to be a basic bare bones setup I would say go smaller.

Although, I have seen some really cool 4x2 enclosures on here for BPs, but they have a ton of accessories that make it seem very realistic.