Leaves yellow 4th week flowering

[Cannabis23]

Hi, first time growing cannabis. I'm growing Northern lights photoperiod.

I'm in the 4th week of flowering and the fans leafs are turning yellow, is this normal?

 

[Cannabis23]

Photo below

 

[Beachbumm]

Eh... It's a little early in my opinion. what's your medium, what are you feeding at what pH & ppm and what were your last runoff readings?