I wouldn't add any more. 4 full grown oscars are going to need every inch of that 110 gallon.
Add a pleco, clown loaches, bichirs, or other bottom dwellers if you want more fish in there.
4 Oscars in a 110 gallon is already probably too many Oscars. They are extremely messy fish and you will need all the filtration you can get when they are full grown. Also, if you have a glass tank they can hit it pretty hard and weaken the seams. I had that happen to me and one morning woke up to have of my tank on my kitchen floor.
i agree right now you look at your tank and you say gosh i have so much room.. bur those O's grow fast .. in a few months you will be saying now i know what they were talking about.. besides Os need plenty of swimming room and the tank needs to be kept up to par to prevent hole in the head .. so to keep your oscars healthy it is best to stick with pleco or another bottom fish..four oscars you have to hope they all get along..but that won't be a problem for awhile..