Part 4 of the JoJo series takes place in the Japanese Town of Morioh, where an unknown villain has been using the bow and arrow we saw back in part 3 to give people Stands.
Morioh is also the birthplace of our new protagonist, Higashikata Josuke. Joseph Joestar's son and alas, Jotaro Kujo's Uncle.
The stories centers around finding the arrow and the person or people who have been using it.
Most people will agree, part 4 is where JoJo really transformed into what we know and love today. Moving away from the tarot-themed powers and fully into the musical references for it's characters and their stands. In doing so, araki also leveled up his creativity, and some of the most bizarre stands can be found in this part of the manga.
This is also where we meet one of the series's most beloved villains, Yoshikage Kira.
Highly recommend this reading this instead of watching the anime, and experiencing the artstyle changes directly.