Make no mistake; the first three parts are entertaining in their own ways, however they are noticeably dated and lackluster in characterization and storytelling nowadays. While experiencing the first 3 parts, I was entertained certainly, but I wasn't that emotionally invested.
Then comes part 4 and it actually manages to catch me by surprise at how much better the writing becomes.
Part 4 takes place in Japan once again, this time in a small town called Morioh. The catalyst of the story is Jotaro coming to the town several years after SC to find the illegitimate son of his grandfather, a teenager called Josuke Higashitaka. Unsurprisingly Josuke turns out to have a Stand, and from that point onwards both of them get caught in shady dealings and eventually discover a grisly series of murders and try to find the killer.
What really makes part 4 shine is that unlike the previous parts which were more or less just pure action, it focuses a lot more on the character development, and manages to make even the side characters surprisingly lively and memorable. The protagonist, Josuke, is a huge improvement over Jotaro, with a much more interesting Stand and is overall a much more memorable and charismatic main character, similar to Joseph in ways but different enough to stand on his own. The supporting cast is great, from the wimp-turned-badass Koichi, the friend-turned-foe-but-still-just-as-dumb Okuyasu, the conceited and insufferable genius artist Rohan, and of course the unforgettable main villain Kira Yoshikage, who is one of the best antagonists in the entire series.
The fights and abilities are generally much more creative and interesting than Stardust Crusaders; Araki seemed to realize how open-ended the concept really was and strove to live up to the title of "Jojo's BIZARRE Adventure". the final fight with kira is one of the best moments in the entire series.
This is also the part where the series' art starts developing its own identity; while the earlier volumes keep the fist of the north star look, it eventually transitions to a more realistic style with a much more extravagant and fabulous art direction, which the series is now famous for. the art in the later portions is legitimately pretty nice, easily some of the better art Jump has had in its lifetime.
As good as Diamond is Unbreakable generally is however, it does have its problems. Mainly, while the characters are great, it seems to shelve a lot of them aside as time goes on, squandering their potential. Once the main plot with Kira comes in, a lot of them just stop mattering. Araki also ran into a problem with Jotaro's stand being too overpowered and it just feels too convenient that he isn't around most of the time to solve everything for the sake of keeping dramatic tension up. Additionally it does take a little while for the "main plot" to actually start; while i found it very enjoyable to read up until that point, that is when it truly begins to shine.
Regardless of its problems, part 4 is easily a big improvement over the other parts in the areas that matter. Starting with DIU, the series would generally get a lot better and the stories are no longer dated and bland.
