It's not a matter of if, it's a matter of when. Hasbro obviously is going to want to do more movies and new post on the forum and around the net have shown that. The thing is, Michael Bay wants to take some time and do some low budget story driven movies. He has his money from these three movies and his production company that did the Friday the 13th and Nightmare on Elm Street remakes. So he's not in need of money and can take the time to do what he wants. So even though Hasbro may want him back, their reality may just be that they will have to go with someone else. Which leads to a problem. There are directors out there who can take over this franchise and maybe even surpass what Bay has done. The problem is that Hasbro wants to put out a movie and they want to do so because it helps with the sales for the Transformers toy line. We have to hope that with them wanting to put something out that they don't rush to put something out and we end up with something really sub par.
I think they should take some time, maybe a year or two, and decide if they want to continue where Bay left off or reboot. Then from there get some good writers who are good with sci fi, action, and the fantastical. Get a solid script and then find a director who can best bring it to life.
Also, since Sam's story has already been told, if the 4th movie isn't a reboot I think it should take place in the future. Not far in the future. Maybe just 20 or 30 years. Maybe the world is a better place. The Autobots have a place in our society and more Autobots have come from space to make Earth home, or maybe even colonized the moon. They've shared some of their tech with us. Maybe we learn from them and try to keep what happened on Cybertron from happening here. There's peace, it takes work but there's peace. Then one day at some space monitoring center, readings come in of a small city sized craft approaching the Earth and it's giving off energon readings. I'll leave it at that.