Greetings all. First time grower. Recent major yellowing in need of help :(

Hi guys, I have recently started an indoor 600w grow with vanilla kush. It is my first grow... And the plant has slowly but surely started to go from light green to yellow...2 of the 4 plants even have brown spots on top of the yellowing leaves. yet one of the plants is actually fine. I am in need of a guardian angel who canvhelpcme with some well needed advice. I am not sure if it is nitrogen deficiency? I am on iPad so cannot upload pics on here but would be great to communicate with someone via email or bbm? Hopefully someone will help me as I really do love my new found hobby (consumption only) Joe

Any help at all would be fantastic

hey man, how old r plants and what r u using for dirt/nutes?

They are 4wks... Coco based... And I only use canna coco a and b at the moment as I thought it would be best to keep it simple in the start? I can't think of what it can be ?

The pictures are available in my other thread if you would be kind enoivh to take a look and give an opinion....?
https://www.rollitup.org/marijuana-plant-problems/499591-first-grow-4wks-yellowing-leaves-2.html

hey dude i started recently too i had the same problem, what are your temp and humidity levels, whats your space like (fans, lights, tent or cabinet)