I'm not sure being able to work the 4th ring would actually be that OP. You'd get some extra resources/luxuries to grab, but that's about it. I mean you'd have to have an insanely large city to work all those tiles, and it would be very hard to grow it fast enough too do before the end of the game. I mean, you need like a 35 person city now to work all the tiles with 3 rings, and that's even without any specialists. It's pretty rare that I even have more than one or two cities above 30 by the end of the game.