Neither of these is Part 4's main plot. They're just part of the story. The main plot is "weird shit happens in Morioh", since Morioh itself is kind of the main character. Kira is just part under that umbrella, same as his father, same as RPS Kid.
No Morioh is the setting, not the plot or one of the thread that moves the narrative forward
PLOT THREAD 1: Jotaro comes to Morioh to meet Joseph's illegitimate child Josuke.
PLOT THREAD 2: Tons of disappearances are happening in Morioh! Why?!
-Jotaro & Josuke meet (PT 1 partially complete)
-Jotaro and Josuke fight Angelo
-Angelo tells Jotaro and Josuke about the Bow & Arrow
PLOT THREAD 3: Find the Bow & Arrow
-Josuke & Koichi meet and fight the Nijimura brothers
-Backstory & character development for the Nijimura brothers.
-RHCP appears, kills Keicho and steals the Bow & Arrow
PLOT THREAD 4: Find RHCP and reclaim the Bow & Arrow
-Koichi and crew encounters The Lock and Love Deluxe - lots of character development
-Josuke & Koichi encounter Surface (kind of a shitty episode)
-Italian Food episode is irrelevant to plot but its fun and is a bit of world building
-Josuke & crew encounter and defeat RHCP (PT 3 and 4 complete)
-Josuke & Joseph meet and bond (strong character moments for both & PT 1 fully complete)
-Josuke & crew encounter Rohan - strong character moments for Josuke
-Josuke & Jotaro deal with fallout from RHCP and bond (strong character moments for both)
-Rohan and Koichi encounter Reimi who tells them about "The Murderer" and the threat to Morioh
PLOT THREAD 2 revised: Morioh is under seige - find the murderer!
-Kira is introduced in earnest
-Josuke and Okuyasu encounter Harvest (honestly the primary goal of this arc seems to be to introduce a character of importance to die later on to raise the stakes)
-Yukako and Koichi encounter Cinderella and bond becoming a couple (also set up for Kira's later escape)
-Shigechi encounters Kira and is killed. Killer Queen is introduced.
PLOT THREAD 2 revised: Kira is the killer!
-Morioh Crusaders strengthen their resolve against Kira
-Jotaro & Koichi encounter Sheer Heart Attack
-Kira barely escapes and changes his face
PLOT THREAD 2 revised: Find and defeat Kira who now has a new face!
-Josuke and crew investigate Kira's house, learn about his past and encounter his dad
-Another Bow and Arrow is revealed to exist and is lost when Kira's dad escapes
PLOT THREAD 5: Find Kira's dad and the second Bow & Arrow!
-Kira gets used to his new life
-Rohan meets and defeats RPS boy
-Josuke & Okuyasu meet an "alien"
I mean I think its pretty clear what is moving the narrative and what the current/remaining plot threads are. The weakest arcs have been Surface and the two most recent ones. As I said Harvest struck me as a "lets introduce this character so we can kill someone later than people might care about but aren't part of the main crew". I didn't hate the actual episodes but I don't know if I can say straight face that they were especially awesome. Shigechi was also being kind of a punk. Perhaps it was all worth it though because I thought Shigechi went out like a trooper in the end.The RHCP arc also ended up being a pretty big disappointment because the build up was so strong but then the pay off was weak as hell. It was also pretty isolated from PLOT THREAD 2 when I expected it to build towards that thread or fold into that thread. Nitpicks aside, and I have a good number, I've enjoyed all the other arcs - some quite a lot.
Right now, the current goals are "find Kira" and "reclaim the second Bow & Arrow". Again not every episode has to move those goals forward but if an episode doesn't AND I don't find it fun/interesting then there isn't much point to such an episode. What do we tend to call episodes that don't really serve a purpose and aren't fun or interesting to consume.........?
Remember when Koichi had an entire scene wondering about the disappearances in Morioh? That was, and is, the main plot.
Kira's not like Dio or Kars. In the previous parts, everything traces back to the Pillar Men or Dio. Kira's the main villain, but he's not the driving force of the story. Kira didn't create a single Stand user, the stand users his dad creates aren't his minions. If anything, Keicho was the one who kicked off the plot.
That's what irritates people so much. It's not about criticizing or expressing disappointment in episodes, it's about treating the story as if everything has to relate to Kira.
At that point, the conversation becomes "That's not how DiU works", "But it's filler!", "No it isn't!", and so on and so on. That's not a discussion, it's not even interesting criticism. It's just a meaningless thread-shitting back and forth.
Did you read what I wrote? I literally said:
"Not every episode has to work towards those aims or provide insightful character development but if they don't, they better be fun/funny/exciting. IMO, the previous episode didn't meet that bar and the most recent episode was kind of meh."
I liked the Italian Food episode even though you could cut it completely and the plot wouldn't be affected at all. To me, it wasn't a waste of time. The RPS episode felt like a waste of time to me. That's the critical difference. How is this hard to understand?