Can't get my USB-C to Display Port Adapter to work on new iMac; works on MacBook Pro I bought a new 2017 iMac 27", but I still want to use my 4K monitor. I was using the monitor with my 2012 MacBook Pro just plugging in the mini-DP cable into a Thunderbolt 2 port. 

I tried the "All Smart Life" USB Type C to Mini DP adaptor, which advertised that it worked on the new MacBook Pro, but it didn't work on my iMac.  

Does anyone know what adapter I can use to connect my new iMac to my 4K monitor?
Thunderbolt is a "multiplexing" of several signals including DisplayPort so the signal already exists there.  You just need a Thunderbolt to DisplayPort cable that goes from USB-C to DisplayPort (or mini DisplayPort if necessary) and you will be able to connect your 4K monitor.