HDR is largely software enabled. That's why the original PS4 can support it after a software update and similarly PC graphics cards can too after a driver update. You are right though that HDR will be only visible on newer TVs/Monitors.

However we've had color and lighting effects on PC games for years that are not dissimilar to HDR and that's what I mean by 'HDR type effects'. Also most games even as old as Unreal Tournament can be natively cranked up to 4K resolution if you have a display that supports it. A lot of graphics cards from years ago have been able to do so but as you mentioned, the displays to fully take advantage have only just become available.

For PC gamers, we have been playing at higher resolutions than HD 1080p (1920x1080). About 6 years ago, I bought a monitor that does 2560x1440 for example. Now that 4K has come along, most Graphics cards and games that have existed for a few years now (sometimes more) will allow for ramping up resolution. HDR as mentioned, is a software driven effect so as long as you have the display, chances are your Nvidia or ATI card can already do it after an update.

Thanks about the PS4, I hope so too I just received the message that it has been sent so I am waiting with high anticipation/suspense