It's all going to be dependent upon the setup of your MacBook Pro. What VRAM is on the graphics card? If it's an old or base model one it may only have 512MB allocated, which will possibly mean sluggish playback regardless of which viewer you use.
I have an iMac, very old, that only has 512MB of VRAM on the graphics card, but, if I quit everything else, it plays my 4K footage perfectly, without hiccups. It really does come down to the graphics cards ability, rather than the amount of RAM you have for good smooth video playback. If you can shut down all other processes, you help things along nicely too.
I am thinking 8gb ram is plenty for just playback, ditto on chip video sharing ram.
Editing is a different story. Make sure you have nothing playing in the background, Check systems utilization and if everything is minimal try rebooting before playback. Not sure what else to recommend. Depends how old the MBP is, and if it is “only” dual core or if it is 4 to 6 core or not.