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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Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.