Any suggestions on what you think the problem coul...

The new video and pics are great for describing the issue(s), @TrustNone. Bad news is that it looks like a mixture of quite serious issues to be having in week 4 of flowering unfortunately. Nute burn; lockouts of some nutes (PK and Boron I think), and root issues (very small pot - I suspect root bound issues as well, nothing to be done now though). I would defoliate the fan leaves that are damaged. They're sucking the life out of the plant and she'll be using what energy she has (not a lot as most of the fan leaves are buggered) trying to fix them/use them. In week 4 of flowering, she'll have stopped or very nearly stopped her stretching, which means veg growth will have stopped/very near to stopping, and thus no new fan leaves will be grown by her. This is going to effect your potential yield a lot :( She needs intensive care. Defoliate and flush with a weak bloom feeding solution (6.5ph is fine, make sure the solution is 72-74c). I suspect that about four days after you defoliate and have flushed the media with a week solution (with the run off being very, very similar to what is being put in), she will have unlocked the lockouts and be recovering, slowly. Good luck!