memory footprint of 4k monitor on 2018 13" Iris 655

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.