I have a dead laptop – mobo is gone, it has 8gb ddr3 500gb sata and amd a5000 chip. I was hoping to get a small barebones I could put linux mint onto to supplement the smartness of my not smart TV (eg it doesn’t recognise my NAS with all my kids videos and ripped dvds on it)
I already have a ZOTAC Nano in one room which is ace, but these have gone up in price..
Recommend me a decent box?

I’ve got a Gigabyte Brix which is good imo and reasonably cheap.

A Raspberry Pi3 will almost certainly do all you want cheaper though.

Yep the pi3 works nicely, plenty of ready to go os images out there too

Yeah a Raspberry Pi with a Kodi distro such as Libreelec will do this easily and cheaply.

Would this handle watching 4K YouTube videos?

This alternative says it can
https://www.trustedreviews.com/news/this-20-raspberry-pi-rival-comes-with-4k-and-hdr10-support-2953894
Maybe not
https://raspberrypi.stackexchange.com/questions/44089/can-raspberry-pi-3-do-4k-video

But as suggested in that one a 4k Chromecast will do it simply and easily