Boas have a much slower metabolism than BPs and do not need as much food as BPs do, pound for pound. They should be lean and square-ish in appearance. The rule of thumb with boas is to not feed anything that leaves more than a very slight lump. And adult 6 year old boa should not be eating any more often than once every 2 weeks. I'd like to see some pictures please. 4 ft sounds pretty small for a 6 year old. My 3 year olds are 4 ft.....
And if your hot spot is only 82-85, then you need to raise that up a bit. Boas do tend to like things a bit cooler than BPs, but not that cool. You need to strive for a hotspot of 88-90. Ambient temps are good at 78. When temps and humidity is right, you'll see him a lot more active. :gj: