Just gonna throw my honest two cents out there:
I have a King and a Borneo STP (blood family). I personally would offer more height for an adult colubrid, but a 4x2x1 would be perfect for a blood since they never leave the ground.
I mentioned it in an earlier reply, but my boy Django is in a 4x2x2 and uses every inch. My boy Juice (Borneo), however, would be perfectly content with only 12" of height.
Just food for thought.