First Purge film is fairly meh, it's alright, just standard house invasion shit really.
Second Purge film is FUCKIN AMMAAZZZINNNNN and completely different from the first.
Third one follows on from the second, and is decent, but not as good. Fourth one was a prequel, and is pretty okay actually, was verging on good at times.
New one will probably be shit haha. Actually it's best to watch any Purge film with the thought that "This will be shit" beforehand.