So use a mini roundabout. They are common in the UK. It's just a painted circle with a slight hump, in the middle of a four-way junction. Vehicles can drive over it (and larger ones have to) but it indicates to everyone that they have to give way to traffic from the right and don't have to stop otherwise. They typically aren't big enough for multiple vehicles to be turning a corner at the same time. They fit anywhere.
Yes, and they can be smaller. The circle is about the right size but it has lots of room around it. Imagine a crossroads at the meeting of two residential streets, both just wide enough for two cars. Stick the circle from your picture in the middle of that imagined junction. That's what the mini roundabouts are like on the 1930s suburban estate I live next to.
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:
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.
Here in the UK, we've got lots of roundabouts from tiny mini-roundabouts (some of which have four junctions) that could easily fit almost anywhere, all the way to gigantic multi-roundabout junctions (https://en.wikipedia.org/wiki/Magic_Roundabout_(Swindon) ).
I can't think of a situation where it's more efficient to have four vehicles all stop at a junction (busy four way stop) vs a roundabout which will allow one or two vehicles to join the roundabout without having to stop.
