I think a pre-qual would be good, maybe get Billy back, and show how it lead up to him killing sidneys mum?? maybe showing some history, billy as a kid, then lead up the killing
I can see where you're coming from with how Billy came to murder Sidney's Mum, but unless they suddenly added a whole murder spree that was not even referred to once in the 'later' films, there would be only one murder. Each of the existing Scream films follows roughly the same pattern - the killer bumps off several people secondary to the main plot, before eventually facing off with Sidney. I'm not sure there would be enough action to sustain a full film.
I think a forth Scream could fit in to Randy's "rules". After a trilogy, people expect it to be the end, but sometimes, after many years of thinking their safe, suddenly the horror comes back. As a 4th is brining new life into an old drama, you can expect many shocks, including and of the main characters to die at any point (even at the beginning).