I know people who play Left for dead in 4K and have no problems with it. Not sure why all your older games are bugging out when you switched to 4K, it's just a monitor. Did you change anything else when you got the monitor? Install any other hardware or software? Maybe a windows update that just happened to coincide with the arrival of the TV and that's actually what's messing everything up?
Or have you a multi monitor setup now? If so try unplugging everyone except the TV.
Sorry for the wall of questions lol, it's puzzling me too.
And not at all. Thank you very much for this