I want a gaming pc that can run games at 4k on low to high settings. note, i will accept used parts and yes, ik scattervolt made a video on this but i just wanna see what i could possibly get.
Thats going to be hard. I will refer with some comrades to see what can be done as typically you want at least a 1080 to do 4K but low settings would be maybe a 1070. Thats still about 33% of the cost and you don't want to match it with a weak CPU.
Leaves you $300-$400 for CPU/MB/RAM (and bulk storage)
If you could find any Sandy/IvyBridge (or newer) i7 with a compatible board and at least 8GB RAM (preferrably 16GB), you could achieve some pretty 'budget' 4K gaming.
Even saying that, provided you have an old HDD laying around, and don't need to factor in an OS in the budget:
Then what Barty has suggested will probably be the closest you can get to that. You might be able to get away with the 1070 to save a few more bucks but it does depend on what you want to spend in the long run.
"Low-high" is quite the range. If you fine tune your focus (ie 4k medium) it's a little easier to answer.
Of course there will be variances between titles, but an overall aim of say "medium" should be fairly consistent - with esports being capable at High or even Ultra.