general rule of thumb for caging is square footage = max snake size. So a 6x3 cage can house an 18 ft retic (up to) and a 6x2.5 can house a 15 ft retic... etc etc
So a 4x2 cage can comfortably house an 8 ft tic, and this will include adult male crosses for SD / Mainland - as long as the conditions are met:
1) you do not push food through them
2) they are not a first generation cross - where the male was SD and the female was mainland
3) It is 50% SD Minimum
Heck, my Dwarf Goldenchild is a breeder and full grown right now, and he's barely 8 ft - he's 50% Jampea, + whatever % came from the goldechild bloodline itself. All things considered, he truly is the perfect sized snake.