FWIW if your clown has been living alone for four years, 'he' is actually a 'she'
It's probably kindest to find her a new home if moving her to a smaller tank is the only option. A 15-20g tank isn't near large enough for a maroon IMO. If you're attached to the fish and really want to hang onto her, I'd keep her in her current tank or a larger one if possible.