All
The Galaxy Watch 4 Classic comes in 42mm and 46mm case options and Galaxy Watch4 comes in 40mm 44mm. All of these Galaxy Watch 4 and Watch 4 Classic models work with the same 20mm sized bands. They all use the same pin mechanism on the back of each side of the band to release and attach them to the watch case.
Older Galaxy Watch 3 uses 20mm or 22mm watch bands.