Not so much of a problem more of a worry

Not problems as such but i'm not sure what is causing the yellowing of the first 4 leaves (first two sets of leaves are yellow on 70% of my plants) other than that they look really healthy and are growing at an alarming rate, i know its not nute burn cause i've not added nutes yet, could it be heat stress from the 600w hps? The ph of the soil is 6.8. Some advice please anyone? Any questions just fire sorry havnt provided much info theres a picture here though.

They're hungry start with 1/4 strength nutes and feed them...and work your way up from there...a couple feedings with 1/4 and then a couple with 1/2 and so on...and don't just feed but water also...like this water/water/feed...so every 3rd time you're feeding...the yellowing is a sign of N def...so ya they hungry..lol..good luck and nice looking baby you have there, you might wana think about topping her also...and welcome to RIU...

EDIT: And keep your temps around 70 to 80F with a little fan blowing on them to strengthen the stalks and cool them down...