iMac (Retina 4K, 21.5-inch, 2019) RAM upgrade.

Apple has all the RAM details in this article, including what they will and will not do:

Install memory in an iMac - Apple Support

I asked a local independent provider how much labor it takes for his techs to open and upgrade the RAM in a 21.5" iMac. He said two hours at US$75/hour. That goes on top of the cost of the RAM and the resealing kit. That's a lot.

Maybe we should ask instead what problems you are having, and see if we can help you make it work better for your needs. It is very possible that it is not a RAM issue. In 21.5-inch iMacs, slowness is almost due to the slow factory hard drive Apple insisted on installing. If your issues are slow boot times and slow app launches, that is NOT a RAM issue—it is the hard drive.

Apple won't do it but an Apple Authorized Service Provider might. It will be expensive and may void any warranty left on the machine. If the AASP says it won't affect warranty be sure to get it in writing.

Ask them. We are just fellow users. I don't believe the RAM is upgradeable in this model.

As an addendum to rkaufmann87's comment if you can struggle thru with what you have until early 2021 the new Macs with Apple designed and produced CPUs will be released and that would be worth waiting for.

No, Apple will not upgrade the RAM on a 21.5" iMac. However you may be able to find an independent Apple Authorized Service Provider that is willing to do that. Please note this requires a complete disasembly of your computer and is difficult. To get an idea of how difficult please watch https://eshop.macsales.com/installvideos/iMac-21-inch-2019-memory/.

This means it may be expensive. You may want to consider either selling your current computer or trading it in and ordering a 21.5" with a minimum of 16GB of RAM (more is better) and the largest SSD you can afford.