Yellowing Leaves in Flowering week 4

I would say potassium deficiency.

soil or water ph is low as a result your leaves are showing a deficiency. check soil and water PH.

Like Hashy says plant needs magnesium. If you want the plant to uptake it quick. You can spray some epsom salts solution on the leaves. 50 ppm @ 6.5 ph. You can also feed the roots at the same rate. If you spray the plant do it just before lights go on. Cheers!

Hi there I would say that's a magnesium problem. Gets some calmag and add it to your feeding regime.