Can you answer few questions about finding used laptops that can output 4k?

What are you planning to do with this 4k display?

I hope you're not trying to game at 4k on a laptop, only the highest end laptops can do that, and you don't really gain anything visually unless you're gaming on a huge external monitor, TV, or projector.

But most laptops would be able to connect an external monitor and run it at 4k. Apple laptops have had close to 4k screens since they started using their "retina" display. Just look at the max resolution for the video chip in the laptop's specs, or as P said, an HDMI 2.0 is required to output 4k resolutions.

Basically you need a laptop with a "Thunderbolt 3 " capable usb-c port anything less than that and it's a complete gamble for 4k unless it says it specifically has a HDMI 2.0 port. You will need a beefy on-board graphics chip like NVIDIA GTX 1060 or higher if you expect to play games at 4k.

All PC/laptop makes will state 4K capability, on those that have it. Not sure why you need to know when it became mainstream.

The solution to high speed/graphics lappies and PCs is always the same - get the best one you can afford at the time, so at least $2K (in the USA), 128/256 or 1TB SSD, 1TB storage (if getting smaller SSD), 16GB RAM, i7Coffee Lake or i8 CPU, and GTX 1080 minimum.

Btw, Nvidia released a graphics card at the annual 'what's coming up next' show in Las Vegas recently, and it edits 8K on the fly. However, double the price tag I suggested to buy it. lol. Also, when a new card is released, it's often smart to wait up to 6 months before purchasing it, bc price will have come down (more sold by then).