It's hard to believe that there's still one more film left because of just how perfectly fitting the closing scenes are. This has to be the best of the series thus far, as the relationships we're already familiar with reach breaking points over and over again.
In this fourth movie, the public gets tired of the yakuza war and puts pressure on the police. Panic sets in amongst the gangs as underlings commit unsanctioned acts of violence and bosses denounce their own men. This could actually be the series finale. It really brings a form of closure to the story as every big character is either killed or thrown in jail. It contains some of the most memorable scenes so far too.