I have 4 monitors connected to my computer (yes, I like my screen space 
). I’ve read somewhere on the web that one can stream a multiple monitor setup, but need a separate PiKVM for each monitor, and they will appear in different browser windows. Can the same be accomplished with, let’s say, a V4 mini and a switch? At a first glance it seems so, but I’d like a confirmation before ordering.
No, the PiKVM Switch lets you connect multiple computers, but only connect to a single one at a time. You would need 4 PiKVMs and probably some modifications of the mouse behavior to have 4 different PiKVM “screens” managing the same computer behave the way you’d expect. Also the PiKVM v4 mini is NOT compatible with the Switch, it doesn’t have a spare USB port required to connect to the CTL port of the Switch in addition to the OTG port.
You could try using some video wall hardware to put all 4 screens to a single 1080p stream, but then you aren’t going to be able to read text unless you also have a way to remotely toggle that device between 4 in 1 and switching between individual inputs.