The real difference isn't really in playback. Where you really hear it is when you do a lot ot of manipulation with plug-ins. The more informatuon information they have (i.e. Samples) they have to work with, the better the job they can do. Uses up more processor as well, but lemons into lemonade, eh?
The big one for myself (and a bunch of other people I'm sure) is with time stretching/compression. You can push a 192k sample much further than you can a 48k or even 96k.
But! I wouln't wouldn't immediately rush out and buy a Sound Devices 702 or something because it can do 192. In terms of audible difference of the samples/sounds themselves, I think you'll be hard pressed to hear a difference between even 48 and 192, let alone 96 and 192. At this point, I would spend my money on mics, not a recorder. The H4n's issue is not a matter of preamps, it's that the on-board mics are electret condensers and are naturally noisy. The actual pre-amps for the xlr ins are perfectly decent.
However! That is just my opinion!