1080i/p HD will look as good or better on a 4K TV as it does on an HDTV. These days, I find that a lot of HD programming and channels from BDUs looks and sounds soft, more like a stereo DVD than HD. Even with a standard HD subscription, Netflix typically looks and sounds much better, especially with their in house 4K content.
VHS tapes will look terrible on any TV set. That's due to the source, not the TV. A new 4K set, it will have a better picture and better scaling than any 720p set.