The solutions for homework 4 are online.
I also put a link to the blog review of the course content (as posted by you) from the lecture notes section. I have read each one of them, and I would encourage you to do so yourself so you can get an idea of what you may have missed that others seem to have caught (or vice versa)
ps: Here is an interesting mini-project idea involving collaborative filtering to suggest ideas from course to students
1. Extract structured record from the postings
2. make an studen-topic matrix
3. Use collbaborative filtering to recommend, for each student, one or two topics that they might like because students
just like them seem to like those topics..