4 Pole TRRS Connector from Steelseries undetected when plugged into iMac 21.5" 2017

Yes, it Should be working. you could take a look in the headphone socket, and if you see a red light, then the digital output has been activated, assuming that your iMac does digital audio output via the headphone jack. Otherwise, get audio MIDI setup running, and change the audio output to built in, rather than Line out, or Digital audio out. You can also set the sample rate, frequency, and the audio inputs there as well, should you need to. Also go to system preferences, then sound, and change the alert sounds to play through.....built in.... If you hold down the option key and click on the speaker icon at the top right of your screen, next to the time display, it'll show you all the audio inputs and outputs in one list. If doing those gets you none, then reset SMC/PMU and zap pram, wait for the imac to restart, and try it again. How to reset the SMC/PMU and zap pram can be found on Apple's support page for Desktop computers, or if you're impatient like me, google, yahoo, etc can get that info for you

good luck

John B