Problems in displaying 1080p and 4K

[Nemos]

My PC is connected to 2 monitors (both 1080p) and a Samsung (Series 6) 4K TV. The entire post is about playing video on TV, from PC.
If I use PowerDVD to play a Blu Ray, I do get a 1080p picture on my TV (haven't tried 4K content yet). I have also streamed YouTube, from PC to TV, and successfully got 4K picture (3840x2160). However, if I play the very same content in any of the free video players (PotPlayer, VLC, 5K, etc), I notice that pixel are, effectively, grouped in 3x3 clusters when played on my TV. So that means that all HD and UHD content in PotPlayer gets downscaled to 720p.
PowerDVD is extremely buggy and I can't use it for anything except proper DVD and Blu Ray content. So I need to get PotPlayer to work properly with HD content.
So far, I have determined it's not about the TV, or PC to TV connection. (If it were, it wouldn't be able to play PowerDVD in 1080p).
Does anyone have an idea?

[jagabo]

Update your graphics card's drivers. If that doesn't help try using different video renderers in the various players.

[Artas1984]

What's the difference playing the file from a PC and directly from USB drive connected to TV? Does the TV's processing engine do a better job of upscaling the picture to 4K that PC?