A Switch that runs at 4K, at least in handheld mode, is utterly pointless. The current Switch runs at 720p in handheld mode and trust me when I say that's plenty for a screen that size. When docked it dose make more sence to run in 4K, but I would still rather any extra horsepower a new version of the Switch gets would be utilized to make it more compatible with newer, 3rd party titles rather that satisfy the vanity of sadly loud group of idiots.
So, would would I suggest for a new Switch? More reliable Joycons, 4K media playback while docked, 1080p (max) gameplay in handheld mode and improved performance while docked at 1080p.