I agree that the likely culprit is fungus. That blotchiness is too irregular for N or Fe chlorosis. So don't add N. Anyway that would be inappropriate at this stage because I see that you are in flower already- you say 4 weeks in- so you have 4 weeks left.

With buds on you have fewer choices for fungus treatment, but you can try Serenade or another bacillus subtilis product. Serenade is sold at WalMart.

It is somewhat expected to see old fan leaves yellowing near the end of the flower cycle anyway as the plant uses up its N reserves.

Control the fungus, finish your crop, and be sure to bleach down everything before your next grow. Be aware of your soil pH because a sickly, stressed plant will be less able to resist fungal attacks. Potassium deficiency can do that too.

If by some chance that is a virus, you have a tougher problem. There is no cure for viruses and the plant should be destroyed. Or you can let it finish but be SUPER aware of insects, as they can spread the disease to a healthy plant. Put up sticky traps.

At least you are almost done with the grow!