iMac (Retina 4K, 21.5-inch, 2017) ram

Why are you wanting to upgrade the RAM? If it is to speed up the iMac you're barking up the wrong tree. Additional RAM lets you have more apps open at one time without a performance hit or lets you do more efficient processing of large files.

A faster ppt drove will speed up booting, opening apps and files and saving files.

Let me guess: you have 8 GB of RAM and a 5400 rpm hard drive as the boot drive. Am I correct? If so that is the slowest iMac Apple offered at the time it was purchased.

You would be best served to get an external SSD, clone your boot drive to it with Carbon Copy Cloner and boot and run from it.

I can strongly recommend the external SSDs from OWC (MacSales.com). OWC is the premier 3rd party hardware provider for Apple computers.

Give their Customer Support personnel a call and see what they recommend based on your current iMac's capabilities and your budget.

The 2017 21.5" iMac must be completely disassembled in order to access the RAM.

see > Install memory in an iMac - Apple Support