Just FYI ball pythons can get bigger than 4 foot! Usually not too much bigger, but I don't want you thinking that the 3'9'' ball python is "almost done growing". Also, growth in BPs, since they are a heavy bodied snake, is usually referenced more by weight than length, since it is more important for breeding purposes.
Carpet pythons are composed of a couple subspecies, Irian Jayas being the smallest at ~5-6 foot(correct me if I'm wrong plz) as adults, and coastal carpet pythons being the largest. I currently own an '08 coastalLarge female coastals might break 10 foot if they are especially large, the biggest on record is a wild specimen at 14 foot.
The other subspecies fall in between. I have seen some pictures of particularly large female jungle carpet pythons, but these are certainly not unmanageable snakes. It's important to remember that a 8 or 9 foot carpet python is still quite skinny for its length, compared to heavy bodied snakes such as ball pythons or blood pythons.