You're not very bright and your noobness is showing lol.
Indica plants grow shorter (less space between internodes) or squater than sativas.
So is your noobness.
Indicas do go short and bushy, but his is not where it should be.
I could show you hundreds of indicas that aren't that short at 4 weeks, and don't have that short of spacing.
He has a 600W 6 inches from the tops of them and is wondering why they are unusually short, and that is why.
Plants need light at a good distance, but not too far where it doesn't get enough, and not too close like now, where it produces diminishing returns.
Plants want to stretch towards the light, a foot between the light and them lets them stretch. You can toy around with how far the light is from the plant and see results. Obviously you don't want them to stretch too much, just enough to where they aren't so small that the nodes are so extremely close together that it actually produces less of a yield because of light penetration being a problem. Some people would argue to use defoliation technique in a situation like that, but I would personally recommend to avoid that.
4 Weeks, 6 Inches, and look at his node spacing, it isn't consistent.
Pull the lights back a bit, and if you want, top them at the 2 true node. This will increase your yield by producing 4 main tops, instead of the normal 1.
This will also make your plant increase in height faster, as it starts to grow the lower nodes. Trust me, 7 days after you top, your plant will grow more than it did in those last 4 weeks.
I topped all my plants, I haven't updated my journal with today pictures, but they all grew 1 inch 30 hours after being topped, and are showing new and improved growth.
Keep in mind 3 are 1 week older than the other 3 as well.