Oldest/cheapest iMac to edit 4k footage?

[slackerinabox]

I got a 4k lumix camera and would like to edit footage just for fun/family. I'd like to spend no more than $1,000 for a refurb, how cheap can I go? I don't even need to be able to view 4k, editing by proxy's fine as long as it's relatively simple and real time