How many Acts should we be expecting at release?

I know that the third act is coming in open beta, but is that going to be the last act or will there be a fourth?

Also, thanks for the play time this weekend. It was great, so I purchased a key/100 points. Can't put this game down.

I've heard that the advent of the fourth act will signal the full release of the game. As long as is financially viable, GGG will continue to add content/acts. They have a ten year plan, so...that sort of makes clear that four acts likely isn't the end of the story.