that is an average weight for a normal cat. He probably lost the weight because you are feeding him better food and he is now more active (he was in a cage before).
Weigh him again in a few months to make sure that he isn't loosing more weight but 9 pounds for a domestic cat age 4 years is good.
Also, when you see the vet for his annual exam and vaccinations, the vet will advise you as to his weight range.
Did he seem overweight when you got him? Its not really normal for a cat that young to just loose weight unless he was overweight when you got him and he's now at a better weight.
A cats ideal weight is going to depend on it's build. My adult cats weights vary from 5 to 13 lbs and none are over weight for their builds.
The only real way to know if his weight is good for his build is to have a vet check him --- and if he's lost 2 lbs in 6 months it would be a Very good idea to have him checked