What's up with plant 4

Yeah that’s not sign of nitrogen deficiency anyhow but you’re locking out other nutrients is likely the case - that run off number is way too high for comfort. Some plants can take it and others can’t.. you’re seeing the result of that in plant 4.. Given that you’re in coco, I’d mix up like 5 gallons of feed for it at a low EC of .8 - 1.0 (4-500 PPM) and flush/leech that coco.. basically flush it but with a feed. You don’t wanna do straight water cuz you’ll likely stress it out even more. But you want that run off to be down below 1000PPM before you quit