2 and 4 are males - you can tell from the 'bib' (black and white markings on the chin), rust red feathers on their underside and the blue-gray feathers on their sides. All 3 are things you don't find in females. Females might have a light colored area on the chin, they might have a white area, the area might have a black border - but you don't see the full black and white pattern as on those two males. Not all males have them, though. But unless the male is white on the belly (tuxedo or all white) you will always see rust colored feathers around the vent, so that's my preferred way to sex them. The rust colored feathers will be more pale if the bird is silver, cinnamon or another dilute variety, but they will still be there.
3 is an obvious female, 1 is probably female but the picture isn't the best.
