Fan leaves yellowing at week 4 of Flower

It looks like the leaves here are all starting to turn yellow at 4 weeks of flowing. Should this be happening so soon? What can I do to prevent it if not? Here's some pics. Appreciate the feedback thanks!

Potassium deficiency. Give them a good dose of some organic high-K ferts.

They probably need some N as well.

Im giving them the fox farm trio hydro. Grow big, big bloom, and tiger bloom. Since its 4 weeks im still / have been giving them fertilizer, yet the leaves still yellow.

They look lacking in Nitrogen for the most part. If you're using Fox Farm it's ok to split the difference between grow big and tiger bloom so you have extra N. For example : I use 17ml of grow big and 17 ml of big bloom per 5 gallons at about where you are in flowering. If it keeps em green, go for it. I usually don't ease over to all tiger bloom until around week 5-6.