from my experiences 4" is a bit too young to sex but you can get a good idea from the way they start to act as they grow,my young male i had began to get fairly assertive at around 4"-6",whereas my female didn't,they both were at the front of the queue when it came to feeding time though ! the female even jumped out of the water to take pieces of fish etc out of my fingers ! when older the males are a lot brighter coloured than the females with more pointed dorsal/anal fins,also males are more powerful looking IMO,at 4" the spots are only just starting to appear all over,so if it's starting to get fairly aggressive towards other fish it may be a male but they each have their own personalities,my jags were kept in seperate tanks at different times by the way