2K and Firaxis has in my opinion earned a little bit of trust. Look at what happened with the first DLC from CiVI. They had planned 4 DLC, then there were severe changes in the currency world, and in some areas, they could buy the 4 DLC individualy, cheaper than the set. What did they do, they added 2 more DLC to the set, to make up the difference. If they do release a 4th age, they could do it in several ways. A free patch, DLC Content, Expansion etc. They could follow Victoria 3's model. 4th Age is free, but additional content is paid DLC. As long as they continue to update and support the base game, they can release as much DLC as they want.