Chair of my committee keep adding and changing ideas
My advice is that the dissertation, and therefore the proposal, are the student's work.  It is the student's responsibility to pick a topic and execute it.  Faculty are not usually trained in project management so the student may need to act to keep the faculty member on track.  For example, you can tell the faculty member that their ideas are very interesting, but you need to complete your current project first.

More specific to your situation, a fourth year student should know more than their advisor about the student's work.