4x3x2 is definitely large enough for an adult Bci. The 'set minimum' is a foot of square floor space per foot of snake, and an enclosure with 12' is definitely large enough for even the biggest boa out there. A more spacious cage can be a good thing *within reason*, bigger isn't necessarily better, as your snake still needs to feel secure, which the larger cage you have, the more stuff you need to put in it. 2' of height will be wasted however, unless you plan on putting a shelf, or branches in it to give your snake more ground space