720p should look fine if you disable the slight zoom most TVs do by default (simulating the overscan of old CRTs).
720 lines x 3 = 2,160
That's the same number of pixel rows in a consumer "4K" TV, so each line of detail would be displayed 3 times and it should look better than it does on a 1080p panel with non-integer scaling. Even a so-called "720p" TV is really 768p and the image is displayed with non-integer scaling, so your 4K TV should be able to do it better than any of them.