It’s exactly as Helene said: 5 gallons are good for one pair but too small for two.
A Paro-pair needs extremely little breeding and behavioural space (although of course they use more of it in nature. We talk about the aquarium situation). Allan Brown bred most species in small tanks of 5 liters (much less than 5 gallons). I do in 10-liter-tanks. Clearly, the non-guarding female uses much more surrounding space in nature, driving alien fish and shrimps away. But in the tank a very small space is absolutely possible (not for quindecim which is a strong and quite big species; in that case even my 10 liters are a bit too small).
But in the moment a third fish is present or even a second pair, the situation changes completely. Here, the space needed resembles the situation in nature. Otherwise there are constant little combats and arousal; there is no secure space except for the one guarding male. The other three fish do not show a normal behaviour.
So it’s good that you will separate them into two small tanks. (Or one bigger, but then you probably won’t experience young growing up).