Quote - Supposing you simply copied the P4 runtime onto the P6 runtime, would that work? Or would it overwrite something?

You do need to be careful if you do that.
If you copy the whole Runtime, you end up doing a reinstall.
Make sure you copy only geometries, textures, and (most of the) libraries.

It's definitely safer to link your P4 Runtime.
The one thing that it is helpful to copy over is any morph delta files (in !DAZ and similar).