4 months - in a horse with NO other issues at all - it enough to get a horse in that body condition back to optimal weight, in my experience; however,
sometimes there is something else going on. Might be something minor, and it probably is because he does look a LOT better overall, no doubt!
Worming with something like Quest Plus and having his teeth done are things you've probably already addressed, but it not - those could be the problem.
Try a ricebran, beet pulp and alfalfa feeding mix - you can buy Triple Crown Complete because it already has those in the pellet or you can mix your own. It does wonders for hard keepers. Rice Bran can be an oil or a dry mix, which I used. The beet pulp is something many people swear by. I didn't have luck with it; however, it might be a fluke - but the rice bran and alfalfa pellets do wonders!
Here is a rescue I did:
This doesn't show his condtion as good as it could - he looked worse than this photo even shows.
after about 3 weeks to one month on the above feeding
getting his winter coat, about 2 1/2 months, and getting adopted!