I wouldn't recommend a pair of convicts. They get incredibly aggressive especially when breeding, and you may very well end up with a dead Oscar. Firemouths are better, but I think keeping a pair of anything in a 4' tank is going to be tricky with tankmates. If you go with the Bichirs, you could do just the Oscar alone and a couple of those, they go well with cichlids. If you don't do the Oscar, there are plenty of options, but you don't want to get anything too aggressive for a 4' tank, unless it's going to be alone. Some reasonable choices for a 100g community would be green terror, jack Dempsey, severum, chocolate cichlid, A. Lyonsi, nicaraguense. Those are some options, putting every one of those together would be too much however.