If the newer growth looks fine and it's just the lower fan leaves, it could possibly be just a natural occurrence....(not saying that it is). Just saying because you say you have some going on 4 months of veg....4 months is a pretty long time for them to be vegging indoors....but this is why I say it could be a natural aging process of the plant(s)....unless it looks like something is really wrong....got any pics??? Would really help us help you...

Throwing the age out there because my mother plants lower fan leaves yellow and die off, doesn't mean something is wrong....

If you've checked all other possibilities, then maybe it could be the age....

Just my :twocents:....

CGI::::::