Just 10 minutes ago I responded to a similar issue on RCG. Here is a copy of that response
Way back when... I was trying to decide if I wanted a P3P or P3A. I had a buddy send me raw 4k video in drop box to see if my system would play it. It would not, not even close. Now, not only does it play 4k, I can edit 4k. I did not spend a dime.
Before I bought my P3, I posted about my 4k issues here, and on RCG, and on FB forums. Most responses to my posts questioned my system specs. Here they are, in a nearly 2 yo HP laptop. I did not buy this laptop with 4k in mind. 2 years ago I don't think I had even heard of 4k:
i7 at 2.2 Ghz
64 bit
8 GB RAM
1TB 5200 rpm HDD
Intel HD Graphics 4000
Then all of the wizards of smart told me... not enough RAM, HDD is too slow, and I need a new graphics card.
I am sure all of that would help. But I might as well by a new computer or go with the P3A. I did neither. I bought a P3P and kept my computer
So what did I do? I started Googling and found a bunch of other info. This will be a very truncated how-to fix 4K playback/editing, but basically I followed a few resource management rabbit holes until it worked.
Googling said to Defrag your HDD. Mine is set to automatic. I did a manual anyway, and it cleaned up a lot of stuff
Same with malware and virus software. All is set to automatic. I ran manual checks anyway, found some stuff
Next, Task Manager is your friend. Look at it, and see what is running and using resources.
I use Chrome to surf the web. It is a HUGE resource hog. Shut it down first when playing with 4k. Although now I can edit 4k with 4 or less tabs open
Next I noticed Malware Bites (MWB) (my malware software) was using a lot of resources. I ditched McAfee and Norton a long time ago, but everything I read said they were prime no 4k play issue #1! I emailed MWB about hogging resources. They responded with a fix file. MWB uses a lot less resources now
The next big resource hog was some form of Windows process. I do not remember the name (notamex?) or similar. But it is probably slim and none everyone will have the same issue, it's the method I am suggesting you should try.
I Googled that process. Something lead me to that process indicating registry errors, and to a program called CCleaner as a fix for registry errors. I ran CC and boy did it find a whole long list of stuff. Once cleaned that up, it stopped that process from hogging resources.
That was the last big key to playing/editing 4k on my system that is not supposed to be capable of handling 4k. Along the way I got the VLC advice etc. But now 4k plays in Windows Media player, VLC or anything else I have.
So cleaning up my system, and following the resource hog rabbit holes did it for me. YMMV