Requirements:
iMac 21.5, 4K (2017) corresponds to iMac (Retina 4K, 21.5-inch, 2017).
two options depending on the frequency regarding maximum total RAM memory capacity: see technical specifications:
https://support.apple.com/kb...
RAM bar itself:
As identical as possible to the existing RAM bar:
SO-DIMMs (Small Outline Dual Inline Memory Modules)
Unbuffered and without parity (non-ECC)
Number of poles not specified in Tech Specs: DDR4 SDRAM 260-pin?
https://support.apple.com/kb...
2400 MHz
Same timings as the existing RAM bar (if to stay inside): CL tRAS, tRP, tRCD
https://help.apple.com/imac...
Example:
For the 27'' according to Crucial:
https://www.crucial.com/usa...
to consider:
important the recommendations of Apple in the overview section
https://help.apple.com/imac...
If you absolutely have to upgrade yourself, first take a look at the device and see what is already available.
