Green terror. They can be highly aggressive and grow to upwards of 10 inches.
Is anything else in the tank? What's up with it's dorsal fin?
Adding a fish to the tank will likely be a difficult task. You'll likely experience heavy aggression. Green terrors are very big and aggressive fish. Once they get around max size, I've found them to be extremely aggressive.
You'll want a similar sized fish with equal jaw strength. Stay away from the big boys like texas, red devils or jaguar. And remember your tank size. A 4ft tank is not a whole lot of room for 2 large fish.
Stay away from less aggressive south american cichlids like oscars, firemouth, severum, or geos.
I'll comment again on what I think could work once you answer some questions for us.
1 - What other fish are in the tank?
2 - What is you're water change schedule? Colin mentioned you're water seems a bit cloudy and you're terror's dorsal fin is deteriorating or something took a chunk for it. I'd hate to suggest to add another fish if you already have a problem with the tank. Which leads me to question 3..
3 -What are your readings of ammonia, nitrate, nitrite, pH, and gH
