The options to seem fairly limited though with a 60 gallon aquarium and considering the fish you already have in there, but a tiger oscar might be feasible temporarily. If you did add a tiger oscar, I would not add anything else because even this fish would really push the limits, if not too far already. This also depends on the level of filtration and water changes though. I would suggest maybe smaller cichlids such as perhaps a firemouth or similarly sized fish. Perhaps a problem regarding a firemouth is these tend to be a little more peaceful so the green terrors, especially considering you mentioned they was about four inches, might harass the firemouth too much. Maybe a Jack Dempsey, but then the Jack Dempsey will grow and I think you will have aggression issues when considering the tank size. Realistically, you would probably be better off with no more fish, unless you got rid of the silver dollars, assuming you preferred cichlids... I hope this helps!
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