Sexing your plants early and then back to veg will definately stress them and delay the harvest a few weeks, I'm not sure if it would make a lot of hermies though. For the veg cycle you definately should be on a 24hour light cycle! Give the plants all the light they can get, the 4 hours of darkness is going to do them no better. Yeah if the light heat is enought to burn your hand, it will roast your plants. Get a fan and vent that heat or move the light up.