From a compatibility standpoint: A 4 foot tank whether it has a width of 18" or 24" doesn't have the right geometry to enable large fish to have their own territories and escape from each other. A dominant Oscar will pick on the others non stop, he'll never get bored of it and it'll just be miserable to watch. A 6 foot tank creates an opportunity for fish to escape and get some relief. Its really about breaking up sight lines. Consider this: a 125 gallon tank gives an opportunity for red devils or midas to bread and the females to have some chance of surviving the males onslaught in between breeding cycles. 4 foot (even a 150 gallon) would result in the female always being in sight and getting beat up or killed. If your Oscars don't get along (two males or just one a-hole) then they will be in constant conflict/stress :-(
Oscars are wonderful endearing fish, two in that tank as long as they're compatible would be awesome. You will fall in love and want what is best for them trust me. With good water changes you could even keep a few SDs with them. BUT that Pleco and everything else needs to go or you need to scale down your goals and do a single Oscar with that Pleco or he/she will just be slowly poisoned over time. I couldn't bare to watch that happen to any of my big cichlids personally. 3 Oscars if compatible might work in the 6 foot 125 with lots of water changes but I can't imagine them doing well in a 4 foot tank. 4 Oscars in a 4 foot tank... I would hate to see that, again you'll just be poisoning them slowly over time... I'm sure we all agree that watching these wonderful animals die slowly is the opposite of our goals as fish keepers. My O has a 90 essentially to herself. The only other fish I have in their are 5 female convicts who hide amongst the driftwood and add basically no bioload. The Oscar more than files the tank by herself and eventually I'll get her a bigger tank. your tank is just 6 inches deeper. Same sight lines...