What's the problem with the leaf?

Mmm I’m thinking calcium or magnesium deficiency.. hard to say as they kinda overlap sometimes in symptoms.. looks like you’ve got N excess as well by the looks of the darker green shiny leaves in the video - could be wrong about that but that would explain a calcium deficiency. If your foliage does indeed look darker green and glossy almost I might flush or eliminate N heavy nutrients completely. I’m not familiar with the “bio” line up of nutrients to expand on that further, but week 4 of flower you can pretty much cut N completely outta the equation. Good luck 👍