increasing memory on my iMac (Retina 4K, 21.5-inch, 2019)

Allan Jones is correct. I didn't think clearly for this response.

Adding more RAM may or may not help performance. In order to see if it will or not, we need an EtreCheckPro report. EtreCheck is a useful app to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here - https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack

Hi there,

An Apple Authorized Service Provider (AASP) may be able to upgrade the RAM. An Apple Store cannot. Find an AASP near you: https://locate.apple.com. (Note: You have to change your location in the top right corner of this site.)

You can also read: https://support.apple.com/en-us/HT201191

Cheers,

Jack

Welcome cornelis18.

Rather that getting a lot of responses bemoaning the difficulty and expense of upgrading RAM in a sealed-case iMac, I think we can better serve you if you tell us why you need more RAM. I have 16GB on a 2017 iMac running some fairly demanding apps and it is never starved for RAM.

If "slow" is the reason you want more RAM, thousands of reports here statistically say it is not your RAM but your hard drive. Apple chose to fit underperforming mechanical hard drives to entry-level iMac 4k models.

If your computer is slow to boot and slow to open apps, but apps run fine once open, your have the under-spec hard drive issue, especially with newer versions of macOS. You can throw massive amount of RAM at the problem and not change anything.

We can verify the issue if you post an EtreCheck report. It will not reveal any security or personal information yet will give us hard data of what performance issues and their causes are at play, including if you have the "glacially slow hard drive" issue. EtreCheck gives us diagnostic data on a computer we can neither see nor touch.

Get the free version here: https://etrecheck.com/index

and use these instructions to post the report: How to use the Add Text Feature When Post… - Apple Community

EtreCheck said that the computer is in excellent condition

Yes, EC can say that but then show some potential slow-down issues just below that section under Major and Minor issues.

This gets hard without the report. I had to put on my Guessing Hat. At least copy these lines describing the drive near the top:

Drives:

disk0 - APPLE SSD SM1024L 1.00 TB (Solid State - TRIM: Yes)

Internal PCI-Express 8.0 GT/s x4 NVM Express

so we know what drive is installed. Then scroll down in the report near the bottom and look for this section:

Performance:

    System Load: 1.66 (1 min ago) 1.42 (5 min ago) 1.31 (15 min ago)

    Nominal I/O speed: 0.23 MB/s

    File system: 20.35 seconds

    Write speed:  2156 MB/s

    Read speed:  2863 MB/s

Posting those drive performance scores will not give anything away, and may confirm our suspicions. Those are from my 2017 iMac 5K.

Without being able to see or touch a computer with issues here, we are pretty much at the end of what can be done here.

If you'd like you can upload your EtreCheck report and we can see if there are ways you can improve your Mac.