The first three Purge movies are okay in my book. The story lines are decent, although the violence is definitely too much for me. I spent a lot of time in those movies with my hands over my eyes, and I expected much the same of this one.
This movie is a true prequel, you do not need to have seen any of the other three movies for it to make sense. In fact, if you haven't seen them, I would recommend watching this one first. The First Purge explains more of the reasoning behind the night and follows the story of the first ever purge. Before it became a countrywide event, it was an experiment held on Staten Island. Residents were paid to remain on the island for the duration of the experiment, and the compensation grew higher if they actually participated.
The story follows a few specific characters, who we root for as the audience. It wasn't as predictable as I expected it to be which I enjoyed. There was definitely plenty of violence that had me cringing and closing my eyes, which I suppose is what most people want from this kind of movie.
If you enjoyed the first three movies in this genre you'll enjoy this one. If you like an action movie with a decent storyline and plenty of gunfire this one is for you. If you prefer movies with more heart and less blood, you might steer clear of the whole genre.