750Ti's HDMI is only v1.4 so only theoretically good for 4k30, although nVidia added a driver hack that allows 4k60 4:2:0 with only a few odd flickers in things like Control Panel. So the Displayport is best even though it's not Freesync capable (requires v1.2a that arrived on GTX 10xx)
Gaming performance isn't that bad at 1080p lowest settings for most of the latest games, which should look fine on a 4k monitor no matter how bad its scaler is. The card is clearly more comfortable at 720p with such games though and that should look OK at 1440p
It's actually high res video that's the biggest problem unless you have a powerful CPU (Athlon II X3 won't cut it), as it can only fully hardware accelerate H.264. Youtube 4k is presently VP9 (which requires GTX 950 or newer to hardware decode, 1030 works too but is slower in games than 750Ti) and using H264ify plugin essentially removes all high resolutions from Youtube. It has H.265 hardware assist but there's no H265ify because Youtube doesn't use it. Of course Google is moving to AV1 which would require RTX 3xxx to hardware decode, but none of those cards fit into 75w