How to make 4K video playable on older devices

[nuipiv]

I have downloaded 1080p, 2k, and 4k videos from YouTube
When I try to play it locally, it lag when resolution is above 1080p
Then I try to convert the codec to x264, it improve but still lag when resolution is above 2k
Anyone here has the right config for x264 to make 4K video playable on older devices?
I believe we can customize the codec for x264 but I dont know how

[ProWo]

It makes no sense to watch a 4k video on an old device.
Convert it to 1080p max (possibly also 720p), with a bitrate of 5MB max.

[jagabo]

Why bother transcoding when you can buy a modern media player that can handle 4k for as little as US$20?