You are right, male bettas are solitary fish and although they tolerate other life in the tank, those fish are not 'companions'. They are just fine by themselves and I'm glad to hear you have a real tank for him anyway.
However, I have found in my experience that in a tank with other life in it, bettas are prone to being more active. More reason to roam their territory and keep an eye on things and the activity of schooling fish in a tank can actually make them feel more secure because their comfort and openness gives an idea of safety. Believe me, there is nothing unfair about keeping properly chosen fish with the betta and it can really bring life out of them, if the tank is big enough.
4 gallons is not enough to start mixing anyway, you really need a 10 gallon tank or bigger. Although bettas get on with other fish, they are territorial and we need to give them the right space, and trying this in too small a tank can easily result in aggression problems.
FYI my last betta lived 5 years after I got him in a 15 gallon community and was very active. My new one lives in a 10 gallon with some corydoras and he is even more active.