I just found a tire on Tire Rack that's pretty close to a 600-16 (185mm wide) that would probably work on a 4" wide wheel. They do caution that it's a summer only tire and shouldn't be driven at freezing temperatures, because the compound can crack. To me that would be a definite no-go, but may be an option for guys in warmer climates like SoCal, Texas or other southern states.
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