There are two different 20" wheel sizes.
The international wheel/tire standards use the tire bead diameter as the measurement. The 406 is a 20” tire size that is extremely popular with kids bikes and most 20" folding bikes. Virtually all BMX and 20” kids bikes in the US use the standard 406 tire size. Only a very few Racing BMX bikes use 451 rims in the US.
There is also a 451 diameter tire which has come on a few bikes that is also called a 20” tire. The 451 rim is about ½” further from the axle than the 406. The 451 rim is usually quite narrow.
Nearly all Dahon 20” frames will work with either 406 wheels or 451. Usually the V-brake braze-on mounts on the Dahon bikes are set to work with 406 wheel’s rim brake track. The single hole directly above the center of the wheel on the frame will usually allow you to mount a road bike caliper brake that will adjust to the 451 rim brake track.
The Gear ratios with 451 wheels will be a little higher than with 406 wheels because of the slightly larger wheel diameter.