I’m not sure if this is the same issue as what you are experiencing but when I playback 4K MP4 videos taken with my Mavic Pro on my home PC which is a cheap HP Laptop, they are terrible - not smooth at all.
The same videos played on my work Laptop (fast Intel i7 with plenty of RAM) are beautifully smooth - perfect. They also run nicely on my 7 year old Samsung Smart TV.
Also check your frame rate
On my i7, 24p videos play with a stutter on my external 60hz display, but play smoothly on built in 120hz display.
30p videos play smoothly on both monitors.