OLED when they fix the burn-in issues will be the next big step.
Otherwise if you already have a good high refresh rate TN model, probably it's not actually going to be worth upgrading from. I know I just went through this and tried.
IPS has that terrible backlight glow, even on higher end models, and I asked around and that is apparently the norm. You can find pictures, most people will put them up if you ask, and it's really not good. VA, at least VA with reasonable prices (I.e. $500ish, maybe it changes at $1k+ but I'm not going to spend that much) is very slow with a lot of blurring and ghosting. And quite frankly, the contrast ratio difference is not at all noticeable. If you put a CRT next to a VA next to a TN, you will immediately notice how great the CRT's contrast ratio is over the other two. VA next to TN is going to be marginal especially compared to that.
As someone above mentions, its also game wise. If you play super modern titles only, and more colorful open world at that, it's going to show more of a difference than playing a game like Fallout 3---the monitor is not going to change older content which generally is not designed to have all that information. This is also true for virtually all TV, movies, streaming, etc. You would need HDR content to take advantage of the color gamut etc. So you'd want to make sure you have a good HDR10 monitor, but none are particularly good and all are crazy expensive. For whatever reason, PCs generally don't like HDR or do HDR that much. Getting an OLED HDTV makes more sense for this as well, since burn-in isn't an issue and the difference is significant over LCDs.
Going back to games, TW3 on 4k vanilla is not going to look all that great. It does have colors of course. But it won't really be that special. You have to mod it up and then it will be a rather marginal improvement. You'll for instance notice in the Orchard inn at the beginning, someone walking outside through the window during a cutscene that you probably would miss on unmodded 1080p. Or maybe you'd have noticed a shape there, but not able to make it out that well. Does this really matter? Probably not.
The viewing angles also really only matter if you're watching with a group, otherwise why wouldn't you be sitting in the middle looking straight on? And if you're in a group, seriously, you want the couch and want to be watching on a bigger HDTV, just go OLED.
But for right now, on PC, most people are probably better off just using a fast TN. It just doesn't have the issues VA and IPS have that MIGHT be mitigated at very high prices. The improvements either makes is so marginal you won't care, but your wallet will. Wait for OLED.
