20x4 inch tyre on 2.0 inch rim?
 

Hi, i have done my research and according to what i have learned rim can be about 50% (i want the narrowest possible) of what the tyre width is (like 2 inch rim for 26x4 tyre). Would the same apply to 20 inch wheel? Would 2 inch rim be sufficient for proper setup? (This is for front wheels of a fat trike) Thanks.

26x4 tires typically come with a 80mm or 100mm rim. As a result, there's no way to put a 2" tire on such a rim, since the tire is more narrow than a rim. Same goes for a 20" wheel, you need to determine how wide the rim is. A 20" rim with a 20/2 tire is not unrealistic.