While it works, it’s not a solution. Using a 20-inch 2005 Apple LED Cinema Display as a second screen with a 4k iMac is insufficient. The resolution at 1650 x 1050 is too low & the refresh rate so slow as to leave ghosts of previous images behind the new ones, quite inadequate for use with Zoom. And on Saturday we have our monthly Apple User Group Meeting, a second screen is desirable. I can use the iPad Mini via Sidecar but that display is far too small to be of any use.
So I've bought a replacement, an Acer 21.5-in LED display with a triple interface: HDMI, VGA & DVI. It ships with a VGA & a DVI cable, so that's one more VGA cable I'm unlikely to ever use. I bought a Thunderbolt 3 to DVI adaptor so I could connect the MacBook Pro to the Cinema Display but now I'm using that device to hook up to the iMac.
A couple of weeks ago I tried using a spare 20-inch Acer LED display but when I hooked it up, I could see it had suffered some impact & the screen was cracked & quite unusable. Had I checked a week earlier I'd have been able to put it out with a bi-annual hard waste collection. As it was, I was able to dispose of three dead printers & a 55cm CRT TV in the e-waste section of the pickup.
The ghosting or retention of faint images of windows previously closed windows is NOT evident when connected via a DisplayLink USB video adapter.