Sounds like you're all set.
Only thing I suggest is skipping the 4x3 AP cage and going straight for your adult cage.
You said he is around 4-5 feet? He should have no problem in a 8 or 6 foot cage.
Granted you might want to add some extra hides if he seems stressed or goes off feed. But IMO you'd just be wasting money buying a 4x3. These guys grow insanely fast.
Good luck man! Retics are the pinnacle snake IMO!
Mike