Can someone explain why i cant play back 4k files?

Woooowwww, from description of your hardware, you must be a professional in some kind of media science......and you can't answer yourself.....now that's a mystery

 

What program you using to watch these 4K files?

 

I am not an expert, watching 4K only on my Sony Android TV, but did you check this list?
https://www.cisdem.com/resource/best-4k-video-player-for-mac.html

 

I find it hard to believe, that those two (hardware) can't cope with 4K

 

Can't be a hardware problem. My SONY TV has some funny kind of android box inside and plays 4k absolutely beautiful.
Maybe OSX, something inside that could cause poor performance?

Have to apologize for my "stupid" questions, but it's very intriguing

 

Read AppLe Support document Using 4K displays, 5K displays, and Ultra HD TVs with your Mac and it will answer your question.

 

Simple answer - Your MBPro is the only one that supports 4K viewing.
The iMac certainly does not.
The MacPro might be good, but your performance sounds like you are limited to single threading from the CPU.
Or, maybe the graphics output is CPU-limited for some reason.

Might be interesting to try viewing 4k video on each Mac, while watching your Activity Monitor/CPU tab. That should quickly tell you how many cores are in use.

 

Decoding a heavily compressed 4k video stream takes a LOT of power and is usually done by hardware (i.e. there is dedicated circuitry for decoding video), which is why even powerful older CPUs ( from before-4k-was-a-thing) seem to struggle badly playing a video that a recent netbook wouldn't even worry about.
So that's a thing to consider.

 

Yeah, the GPU does matter, that's why it doesn't make sense to me. The GPUs in both your iMac and the Pro should offer 4k H264 acceleration.

 

There would also be some difference between the files that play, and those that struggle.

 

I am running into the same issue with my 2013 Mac Pro trying to play 4k MKV files encoded with h265. Ive tried VLC, MplayerX and IINA with no luck and followed all the VLC tweaks I could find. The best performance I was able to find was to change the container to MP4 and play it in IINA but it eventually stutter later on.