Looks like the Rift S's resolution is 1280x1440 per eye, so I can see how that is a huge step down from a 4K monitor.
Good news is if you have a powerful enough rig to run a 4K monitor with graphics settings maxxed out, you should be OK if you decide to get a newer, higher resolution VR headset.
Wondering too, if "everything" being out of focus was an indication of dirty lenses on the Rift, or maybe your eyes weren't situated properly in the headsets "sweet spot"?