are the dimensions of your 135 something like 72" x 18" x 24"? Basically it will depend on the personality of the fish in question. once a cichlid reaches maturity (approx. 6" for an oscar) it can be very difficult to get them to accept new tankmates. however if they're all new to the tank then they may get along fine. my advice would be to go ahead and try, but don't let there be any type of territories in the tank. I'd keep it as bare as possible and put all 4 fish in at the same time. then just keep a close eye on them for the first few hours. if there are going to be any problems you will most likely see them pretty soon. the one thing to keep an eye out for is any type of pair bonding behavior. if a pair forms they may decide to make a territory and it will most likely be a very big territory. that may not leave much space for the other 2 oscars.