In a recent interview with Phil Robb, the co-founder of Turtle Rock Studio, he said that they weren't planning on making another game in the series. They even said that some Evolve has Left 4 Dead in it.
"There's a little bit of Left 4 Dead DNA in Evolve, but it's a different experience,” Robb said. This wasn't the only thing he said. "If you look back at Left 4 Dead there are some things there we felt kind of 'proved out' the theory for Evolve, like the Tank battles being one of them. That's essentially a 4 versus 1 experience, especially if you're playing Versus mode, and we thought, 'wow, that's one of the most intense things in Left 4 Dead. We could take that and expand upon it.' That showed us that kind of thing could work."
To make it sure to the audience he said this about Left 4 Dead: "I think we're pretty much done with it." He eleborated on it with another quote. "It's Valve's thing now. We've kind of gone off in our own direction. So I'd kinda like to see something new being done with it, and Valve always does great stuff, but it'd be cool to go back to it as just a fan and not a developer. But I doubt you'll see anything from us."