The latest Steam update caused some problems with the Mac version of the game. We're trying to fix it ASAP. But for now you can launch the game straight from the Finder.
Basically, right click the game in the Steam library. Select "Properites" and then the "Browse local files" button under the "Local files" tab.
Our next Steam patch is done, but the Mac version is too unstable. We're working on that right now.
No date on the Chapter 4 release yet. Stay tuned.