Numbers 3 and 4 look most definitely males.
Number 2 is quite possibly male but give it a bit more time.
Number 1 may also be male but looks most like a female of the four. Give it a bit more time away from numbers 3 and 4 and see if it develops more positive male attributes or not.
Have a look at the photos on this zeb sexing Mick. You may be able to get more positive confirmation with extra photos.
I can never see the venting with the naked eye but when I take closeup photos and enlarge them, it makes it a lot easier.
http://www.plecoplanet.com/forum/showthread.php?t=10056