So this is more than likely a combination of Over feeding and a salt buildup. You're also seeing a Phosphorus deficiency which could be potentially be caused by a lock out if the pH is fluctuating a lot or if the salt buildup has been substantial enough.

You need to try a flush, or cut back on your nutes a full 50% because you are more than likely over feeding as well.

Why are you using bloom and blossom builders if you're only 4 weeks from seed?
Roots sound healthy , avoid touching them with your hands though.. we're breeding grounds for bacteria.

Change your res weekly as well, or 7-10 days.

Now to think of it, that could be the reason and cause. Not changing your res will raise PPM while, topping it up will dilute it.
Full res. changes are important!

Have fun,
Bob. :smokin: