I'm not sure I'd care that much if Murray wasn't involved. I may have been the only GB fan(atic) that really liked the idea of the rumors of him being a ghost. However, I do think without Murray Sony may ultimately decide on a reboot and not a continuation. And that would be hella bad for Dan, who I believe has his heart in the right place about what the fans want/need from the next film.
I also hope he's not recast. Stick him and Dana offscreen in retirement somewhere... I don't even think it's a stretch for the character.
Whatever happens, I have the Ghostbusters Video Game (2009) on my shelf to the right of GB1+2 because really... that was solid enough to be the third film. And depending on who you talk to the general idea of the story (Ghostbusters: Hellbent) seems pretty close to what the game ended up being. The idea that Hell was starting to creep into real-world NYC and that the busters fight The Devil or some sort of demon in the end. There are cringe bits of dialog in there but for the most part it makes an awesome CGI sequel to GB1+2 as well as ties threads of both films together in a way I never thought possible.
Any future GB film will be #4 for me.