yes, jeffreyMJacques, den.thed is quite correct....Target Display mode is not used anymore, and no current (within a year or 3) Macs support it at alll, so there is no official support...about the only thing that might possibly work is "Duet Display/Luna Display" which is something you have to pay for, eg it's not free.... the price is either $4 or $5 a month, and no, you cannot buy it outright, eg, pay a one time fee, once, done, and it's yours for life...no, it's $4 or $5 a month for ever.... is it worth it??? I don't know....to you, maybe... just so that you can still keep using your 5k iMac with Retina Display for as long as possible.... vs the cost of just buying an aftermarket external display....if you want 4k, you could get a 4k tv with HDMI and use that as your display....i'm not saying it would be super great, but it would work...and it'd be affordable... LG would be delighted to sell you a nice 4k monitor for $300 plus tax, not 5k and Retina, but 4k, and they offer free shipping...for a 5k model, you'd be looking at $1,300 plus tax...5k displays carry a big premium no matter which maker you choose, as they are in high demand now.....so if people want 5k displays badly enough, then they are willing to pay extra to have one....it's sad, and a bit of a wasted opportunity, and yes, maybe unnecessary additional expense, but there it is...you might be able to get a slightly better deal by buying a refurbished monitor vs buying one new... here's a link to an Apple studio Display (5k , 27" for 1049 USB-starting price)https://www.macofalltrades.com/apple-studio-display-27-inch-mk0u3ll-a/...free shipping free 1 year warranty.....you might be able to find something good locally, eg, Kijiji, Facebook Marketplace, even eBay (long shot on that one)
John B