What should I get?

I was thinking a 4x2x1 cage is a little big for hatching no?

And putting a divider up would make it not so big for the little fella.

lol about the whole balls droping thing.

I wouldn't worry about it. Just put in several hides and I am sure the little fella will be fine. We have our 6' male BCI in a 4' x 2' AP cage and he loves it.

One thing I would caution... if your cage has the light mounted to the top, there is often a hold drilled to allow passage of the lights electric cord. My female Hog Island boa figured out a way to push the light off of its screws and then went out through the hole in the top of the cage. I found her as I was scrubbing rodent cages on the other side of the herp room-- she was clinging to the back of the rodent shelves, in full hunting mode. I almost peed my pants! :O

Honestly he should be fine in the 4x2x1 even as a baby, just throw some hides in there for him. As a male he might not even break 5 feet so that cage should be good for his lifetime. IMO a male BCI that isn't overfed won't eat any more food or be any harder to care for than a BP and stay a similar size to a female BP as well. You can feed a 5ft male a single large/jumbo rat once a month and he'll be totally fine. Feeding a rabbit every 10 days...yeesh that is going to be a big BCI haha.

I also lol'd at the "balls dropping" statement.