I grow indicas myself & while they do stay shorter than sativas, I think 6" is way short to be at during week 4 if you haven't had any problems on the way with your grow or still have them.. At the end of my 4 weeks veg im always pushing 16" I veg 1 girl at a time with a 150 watt hps from within 8inches of the top. My girls don't stretch at all during veg!! I'm going to assume you are using soil. is it well aerated ? how often do you water ? Over watering will slow down growth. what size planter is it in ? Your lights, tell us about your setup to help narrow down the cause for this slow growth.