Arnold Schwarzenegger recently told EW that he isn't interested in appearing in the franchise's next film without his longtime co-star. He explained his decision in very definitive terms.
"There is no 'Expendables' without Sly," Schwarzenegger said. "I would never do the movie without him, no."
The problem here is that Stallone is indeed out -- his rep confirmed the news to EW last week. There were reportedly creative differences, and the big-name action star decided to leave the project. Stallone had starred in and co-written each of the series' prior three films, plus directed the first.
On top of the Stallone issue, though, Schwarzenegger voiced other concerns. He said that his character, Trench Mauser, "was not well written" in the third film and he didn't feel like he added any value. As such, even if Stallone were to return, Schwarzenegger wants to see a "very well-developed" part before reprising his role in "The Expendables 4." So far, he hasn't seen a script.
There are a lot of big ifs here that need to be ironed out, especially if the film is going to shoot in August as Schwarzenegger says they usually do. It'll be interesting to see how it plays out. Although there are certainly enough actors in the franchise to carry a film without two of its stars, the upcoming film would look very different without Schwarzenegger and Stallone.