I've done the maths. Basically, Nintendo could offer a 4k capable dock with something close to PS4 Pro gpu's power using a card like Nvidia GTX1050, or GTX1050ti if they wanted to have some overhead. Price would be £170-200, though. However, with that kind of power, technically you wouldn't even need to dock the Switch anymore. They could just make it a normal console that played Switch games at 4k.