I have an Acer XV275K P3, a Mini-LED 4k 160hz monitor, and I'd say it looks pretty good. If games have FSR 2, like Baldur's Gate 3, then I can run them at 1080p and they still look pretty nice.
There is a noticeable difference between running 1440p and 4k, but I'm not sure how that compares to native 1440p since I've never had one. It doesn't look terrible I'd say. I only have an RX 7600 so on games like Doom Eternal I play at 1080p which still looks pretty good. Forza Horizon 5 looks and runs great at 4k though.
I'd say honestly going to a good HDR monitor is a bigger difference than running games at higher resolutions. It looks really nice, mine is MiniLED not an oled, so dark scenes can look weird sometimes (mainly in movies, not games), but the benefit is it can get really bright. And the colors are incredible. HDR doesn't have any performance impact as well, and SDR content will also look better simply because you have a nice monitor.