Yellowing/browning near veins of leaves during 4th week of flowering?

Yeah that 3rd pic has some pretty strange, inconsistent colouration and it's certainly not only low leaves, but considering it's in mid-flower the plant is probably just sucking up phosphorus and other elements from the fan leaves. As tetragrammaton says, could *maybe* use a little extra feed, unless you have pretty clear overdose signs. Unfortunately I got no idea what's in the nutrients you're using (and you seem to be using a fair amount), though assuming you weren't using nutrients, I would've thrown some calcium, epsom salts, a bit of molasses, some phosphorus and citric acid at it given the way the leaves are depleting.