iMac 21.5 inch 4k ram.

Tear it down, completely. There is no ram access door like the 27-inch, and the RAM is located on the back of the logic board which then has to be removed for access.

If you are considering buying a new iMac I'd strongly suggest either just getting the Apple RAM upgrade or stepping up to a 5K.

All 21.5” iMac’s built after 2011, must be dissembled to upgrade the RAM.

No. The RAM in those models is not considered user upgradeable.