I purchased and played the first three, but I'll find a way to play 4 without purchasing it. 2, in my opinion, was the pinnacle of the series prior to 4. 3 is officially the point to where the series began to feel a little long in the tooth. There were some amazing set pieces, sure, but I think the overall formula began to take on a "been there, done that" feeling. I hope they kill the series after 4, or at least put it on a long hiatus.
They are the Summer popcorn flicks of the video game world. That is a good or bad thing, depending on what you're looking for in a game.