How do I upgrade the memory in my iMac (Retina 4K, 21.5-inch, 2017)?

The memory in your iMac is not considered to be user upgradeable by Apple (see excerpt from the article below).

Install memory in an iMac - Apple Support

*Memory is not removable by users on iMac (21.5-inch, Late 2012), iMac (21.5-inch, Late 2013), iMac (21.5-inch, Mid 2014), iMac (21.5-inch, 2017), and iMac (Retina 4K, 21.5-inch, 2017). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorized Service Provider. If you’d like to upgrade the memory in one of these models, an Apple Authorized Service Provider can help. Before you schedule an appointment, confirm that the specific Apple Authorized Service Provider offers memory upgrade services.

Memory in iMac (21.5-inch, Late 2015), and iMac (Retina 4K, 21.5-inch, Late 2015) is not upgradeable.

If you take the computer to an AASP in your area, they will do this.

Yes this will cost a bit more however:

a) This will also preserve any remaining warranty left and

b) It will be done correctly.

Screen size is related to memory and cpu speed in that the 27" models have user upgradable memory and the 21" do not and the 27" models are offered faster cpus than the 21" models. Definitely low end of the spectrum.

FWIW, it is possible to upgrade the RAM but it's fairly complicated. You caan probably find instructions on sites like ifixit and there may even be videos. Not for the faint of heart.

On laptops and 27" iMacs it's still easy to add memory modules. Just not on the 21", bottom of the line models.

I get you aren't happy with the inability to upgrade the RAM on your own. If you want Apple to know, as directed please post your feedback at www.apple.com/feedback. We are all end users like you, Apple does not read or participate in these forums, they are simply our hosts.

As far as "Bottom of the line" it is the bottom of the pricing. The 21.5" iMac are the least expensive.

You can always let Apple know of what they should do in the future.

Feedback - macOS - Apple