To me, buying an adult/sub adult BP (or any adult/sub adult snake) seems just like buying a hatchling or juvenile. I don't really see a difference in purchasing a larger snake rather than a small. Would it be necessary to purchase a larger and more mature snake? I think it would, because once you start getting that large snake into handling and getting it used to your scent, it's sort of like you've had that snake for a while (like handling a baby snake, getting it used to handling and your scent). Besides size differences, I don't see how purchasing a large snake is any different than purchasing a smaller snake. Does it really matter? I remember not too long ago (before I purchased my 2nd BP, a while before purchasing the 2nd BP) I wanted an adult BP. It was a female, and I just fell in love with it. It's size and girth, just amazing. The thing was my mom didn't want me to have it because it's size, but it was such a sweet snake. My female will eventually get that large, so if she knew that then why wouldn't she be all right with me having a large snake? The snake was just a bit over 4 ft. too but was big in girth size. She doesn't take care of my BPs or any of my snakes, I'm the one feeding them and everything so I don't see what the problem is about getting an adult BP.
Though, she does seem to care what the family thinks when I get another snake. I always ask why she cares so much about what the family thinks (family isn't that "snake-friendly" let's just put it that way..) but she just says just don't tell anyone. Even after taking my snake to the vet she didn't want anyone knowing I took it there and have to give it a shot each day. She is so confusing sometimes.