What is the traffic flow rate in an intersection with a 4 way stop? For single lane, since only one vehicle can be in the intersection at once, and probably takes _at least_ 5 seconds to start from stopped and cross the intersection, I'm guessing in the 10-12 region per minute best case, so maybe 600 an hour?

Now if you convert it to a mini roundabout, you can have at least two vehicles in the intersection at all times. I fail to see how it wouldn't be an improvement.

I think you are making lots of assumptions here, like when I say space, I guess you assume it is still perfectly flat and the roads are perfectly aligned? The particular four way I'm thinking about, which really should be a traffic circle if they could blow away some houses, is 65th NW and 3rd in Seattle:

https://maps.app.goo.gl/7KBhbJ9oAvDwrfGN8

So notice we already have problems in a bad alignment of 3rd, and 65th is basically a steep grade, even coming up form the west. I think you could put a circle in if it were flat, even with the bad alignment (or maybe because of the bad alignment), but this hills make a non-starter. It also gets enough traffic that I'm pretty sure they are just going to put a stop light up eventually.