The folks over at iFixit have once again opened up the latest Apple product to see what's inside. Their latest teardown focuses on the 2017 21.5 inch 4K iMac. The teardown gives us a nice look inside the iMac, including some interesting changes that Apple has made to its RAM installations on the smaller all-in-one. Where previous versions of the 21.5-inch iMac saw the RAM soldered to the logic board, this new model features (technically) removable RAM modules.
And with a bit more elbow grease, we shimmy the logic board out of its slot to reveal ... removable RAM? We can hardly believe our eyes.
Yep, those are SO-DIMMs. Two of them.
It should be noted that we shouldn't call this memory "user-replaceable". In order to get to the RAM slots, you need to remove, among other things, the iMac's display, power supply, hard drive, and fan. Given that the iMac wasn't exactly made to come apart, we recommend just selecting more RAM up front if you can when you purchase your 4K iMac. If you're looking at a 27-inch 5K iMac, however, you're still in luck: that model comes with an easy-to-use door on the back that allows you to swap out Apple's included RAM for your own compatible memory modules.