I'm thinking of getting a 2018 MBP 13" to connect two 4k displays simultaneously. I assume that once that happens, the system memory footprint is going to increase in order to buffer these displays, reducing the amount of unswapped/uncompressed RAM I have available for other tasks. Please contribute with practical measurements of memory usage before and after plugging in the extra 4k display.
There's not really any significant amount of extra main memory usage for buffers due to having dual 4k-displays.
Indirectly the total main memory usage might increase because some will tend to run more programs at the same time, have more windows and documents open at the same time, etc. - and that will lead to higher main memory usage.
It is correct that the Intel Iris Plus 655 in the MBP uses main memory for its video memory (i.e. up to 1.5 GB of main memory is shared between the CPU and the GPU). However, it is not actually a specific process using more memory in Activity Monitor, but rather a case that there's less main memory available in total.
The size of that extra memory usage for buffers is so little that it is not really noticeable in common usage. For example a video buffer for a 4k display will take up something like 32 MB of memory. That's less than half a percent of even the 8 GB on the MBP with the least amount of RAM.
Again, the indirect usage might be bigger, because you might be loading up more advanced games than you would have done with the extra monitors - and they might use up much more video ram (for textures, etc.) than just the buffers themselves.