nute burn or major deficiency?

its week 4 of flowering. soil indoor grow. ive fed them 1/4 teaspoon of jacks classic bloom a total of three times since flowering. they have been vegged for 2 months.

r/o and tap 50/50 for watering

they went yellow over the last couple days

I see low nitrogen and a very slight magnesium deficiency.
But it also might be over watering since it was mentioned that it happend in the last couple of days... keep an eye on the lower leaves and see if they start dropping off for no reason.
But they look healthy for the most part, so dont freak out.

When plants start flowering, the lower leaves turn yellow from a lack of nitrogen. Its actually good to have low nitrogen levels in flowering, otherwise you get airy, leafy buds, and weak stems.
Im not sure what the fertilizer profile of Jacks Classic is, but I heard its pretty good stuff so I doubt its a deficiency.
In my personal opinion, I would add a little kelp meal to the top of your soil, also add a little cal/mag to your next watering. From my observations, its always better to end with higher potassium levels, so adding a little kelp meal with your jacks classic should be ok. Double check with a ppm test tho. You want to stay around 700-900ppm depending on your grow room temperatures.

So for sure hit'em up with a little cal/mag, then let your soil air out a little, try and watch your plants real close in the next 4-5 days and see if you can catch them just start to wilt from no water, then give them a quart of water and dont water them again tell right before they go limp (giggity).
Tho, if its not over watering, its lack of nitrogen and thats nothing to be concerned about

Good answer Chile. IMO, if he lets it dry out too much before fixing the nitrogen problem he will loose too much leaf. Don't water today, add kelp meal tomorrow with a light watering, then let it dry/air out.