I would say it depends on the individual dovii. At 4" in a 55g tank, you could probably keep him with similarly sized aggressive cichlids depending on the individual personality of the dovii. Red devils, convicts, trimac, etc. as long as they are around the same size. I would not add one or two fish at a time, I would add three at once and completely change the décor of the tank. This will hopefully trick the dovii into thinking the tank is not his territory before the other fish can get established.
How long this would work in a 55g is hit or miss; it might not work now at 4" or you might be okay until the fish add a couple inches. With how quickly these fish grow I would say youd have 2-3 months max before needing a much larger tank.
In the future, as others have stated, it is VERY hard to get anything to live with an adult male dovii. Even if you keep him alone, he will need a 200+ gallon tank. If you had some 500 gallon + tank, you might be able to keep other adult aggressive cichlids with him. Key word there is "might".
All in all, I think you are better off keeping him alone.