And if you don't want an actual Boa constrictor, there are other types of boas such as the Rosy, Rubber and Sand boas that stay fairly small. The various Candoia genus boas stay small as well.
Nicaraguans also stay pretty small... 4' is big for most males.
But yeah, If you dont actually want a constrictor, Sand boas, rosy boas, rubber boas, and pacific tree boas (all already mentioned) all stay pretty tiny.
4.5'-5' is average for a Nicaraguan male... 5'-6.5' is average for a female. Still relatively small compare to the more southern locales. If we are playing the 4'-6' game, let throw in some Mexican boas as well.
I have a 4 y/o female sonoran boa...calm, cool and curious disposition (although she would hiss a lot when she was young, but never tried to bite) and at right under 6' and slender, a very easy size to handle. I could probably bulk her up a lot more if I wanted to, but I don't feed her very large prey so she's got a very "athletic" build as opposed to being very heavy like a lot of them you see out there...