Congratulations for surviving your third year of graduate school! Fourth Year is a good place to be - done with classes, done with the Qualifier, Master's Degree in the bag, and focusing full time on your thesis research. The next thing for you to think about is the Thesis Proposal Defense.
The purpose of the Thesis Proposal Defense is simply to show your advisor and a small committee of interested Astronomy Faculty that you and your advisor have scoped out a thesis project in some detail. You should have significant thesis-related research done before giving a Thesis Proposal Defense, perhaps a publishable paper in the works and/or a presentation at a scientific meeting. And - importantly - you should be able to lay out the directions that you will be exploring over the next 2-3 years. In effect, the Thesis Proposal Defense is like a movie teaser: sketch out the plot, highlight some of the exciting scenes, and lure the audience back in 2-3 years to see how it all works out!
Most of you should be ready to give a Thesis Proposal Defense either this semester or next - talk to your advisors to work out a timeline that you both agree to. The format is flexible, but consists of a talk that you present to your committee followed by a question and answer session similar to what you did for your Second-Year Projects. It is customary that the committee members also be committee members for your final Ph.D. thesis defense. A good reason to do the Thesis Proposal Defense early is the small pay raise that comes with its successful completion.
Sign up for ASTR898 with your advisor before your successful Thesis Proposal Defense and ASTR899 afterwards.