I bought a Yuneec Typhoon H Pro drone on March 18. It's 4K videos wouldn't play well on any of my computers, and they have 4, 4, 8, and 12 GB of RAM.
Finally, I had my business partner, a sophomore computer science major at Berkeley with a 3.96 GPA, build me a super computer with 32 GB of RAM.
This computer has no problems with any 4K video in any video program so I'm thinking that you don't have enough RAM.
However, I think computers with only 4GB of RAM probably have slower core processors, too, so you might need a complete upgrade in all respects.
I don't know whether you're Windows or Apple, but with Windows you can check your RAM and CPU usage by hitting CTRL + ALT + DELETE to get to Windows Task Manager.
Once that comes up, click on the Performance tab and you'll see CPU Usage and Memory Usage.
When I was trying to run 4K on my 4 GB computers, CPU usage was in the 50-60% range and Memory usage was pegged at 3.91 GB.
On my 8 GB computer, CPU usage was at 41% and Memory usage was pegged at 7.91 GB, seeming to indicate that the Windows operating system itself needs .09 GB of Memory.
On my 12 GB computer, CPU usage was at 37% and Memory usage was in the 11.5 GB range.
With my new 32 GB computer, CPU usage is in the 6-11% range and Memory usage has never gone above 16.9 GB, seeming to depend on the length of the video.
Here are the specifications on my new super computer:
http://www.uglyhedgehog.com/t-461559-1.html