leaves look like dying...help?

So i built a new grow box out of an 18 gallon storage bin and have 3 lights hanging from the lid. it also has an exhaust fan toward the back blowing out hot air. I added the fan is added where the red box is.

So now part of the leaves are turning a yellowish-brown. what is causing this is it light burn or nute burn or what? I don't think its nute burn because these plants just started doing it in the new box. They are already 4 weeks old.

looks like they were burned from the light being too close.

def looks light the light burned them it happened to me a couple times

Ok, thanks! That's what I thought it was. I moved it a few inches away, will they turn green again or will they stay that way?