No official cast, premise or start-date has been set for the fourth Expendables yet. The series has not typically followed a rigid continuity, and has mainly aimed to keep the regular cast returning for more sequels. The third film closed with the implication of the older and younger teams uniting for future exploits, but no immediate setup for a sequel was proposed and the film underperformed. It is unknown whether or not Stallone will return for writing and/or directing duties, in addition to being part of the casts in The Expendables 4.
The Expendables 4 is tentatively set to arrive sometime in 2018.