actually there is a way to tell the difference between the sexes without having to wait for a piar or using the old stand by of the fins.
on the female Jack Dempsey the blue marking of the body and face are more on the gill cover and lower part of the face while on a male the blue runs more through the body and the upper half of the face.
here is a pic of my male, the flash has washed out some of his color but you can see that the blue on his face is more of the upper face and into the body compaired to a female.
to vent a fish you would have to take it out of the water hold it upsided down, and between the anal fin and pectoral fins is the vent. that is about all I know about it, I don't vent my fish, as I am not a pro at it and don't want to harm my fish, while trying to figure it out.
I just wanted to add that with small cichlids that it would probably be best to have some sort of magnifying glass avalible as it would be hard to see what you need to on a small cichlid
I have a beautiful male. Males are usually a lot more colorful and have the long trailing fins.
Its really best to let them grow to determine sex. Since you have 4 of them, chances are very good that you'll have one of each sex and not 4 of one.