I recently attended the ADHS 2017 conference. I have attended many conferences over the years and chaired many panels. These are my observations about how to have a successful panel at an academic conference. As chair you have to keep time to insure each panelist speaks for the same amount of time. Unfortunately there is now way to compel someone to stop talking. People have to police themselves; in my experience some are better at this than others. If the chair does not keep good time it is unfair to everyone. The speakers and the audience all suffer. It is rude if you are one of the speakers and you go way over time.
The chair should definitely have all the speakers talk and then have questions and answers at the end. Allowing questions after each speaker exacerbates the time issues and very often leads to the final speaker either getting very few questions or the whole session goes over. Making sure there is time for questions and answers is important for two reasons. Without some interaction between the speakers and the audience the session becomes a completely passive experience. Also speakers like to engage with the audience.
Four speakers at a session is too many and makes time more of an issue. To maximize time the chair should not make a statement. Maybe panels should have only two speakers but that would probably lengthen the conference.
The ADHS conference at Utrecht did have some time issues at some sessions but as always it was a very interesting experience. Utrecht is a great city and I had a very nice time.