I've come to realize that apart from the 4 seasons thing, everything about FH4 is uninspired. From the dialogue, to the businesses, to the houses, it's all... meh.
The "new car smell", so to speak has definitely worn off for me and we haven't even gotten to the second expansion. I've never burned out on a Forza game so fast.
Somewhat opposite experience for me. This is the most time I've spent playing a Horizon game in the entire series, so far. I've mostly moved onto other games on a day-to-day basis at this point, but I still go back for seasonal stuff and the occasional multiplayer every week - it's been like 4.5 months, after all.
Which isn't to say I think it's even close to perfect, of course. I share some of your complaints. I think the dialogue and "characters" are about as uninspired as it gets, in terms of writing, and the extremely surface-level implementation of them. I think the "Horizon Stories" format is undercooked, and mostly boils down to unskippable blather, followed by a basic time trial. I wish they'd done more to extend themselves a little more, into doing MORE with the Horizon concept, beyond adding cosmetics (which I'm 100% fine with them adding, by the way - they are leaning into the more laid-back, casual feel). The multiplayer has issues (especially before FFA Adventure came in), that I needn't rehash here - we all know them.
It seems pretty clear to me that trying to implement and solve the technical hurdles around the "seasons", and the associated work with having to account for four versions of the same map - not to mention the switch to a more TDU-style live environment with real players - ate up the vast majority of their development time.
At the same time, the driving is still as good as it's ever been. I think this is my favourite map of all four games, in terms of verticality and layout. The actual core racing is still a benchmark, for me, among open-world racers. I ultimately can look past all of the half-baked stuff, because for the most part it's still just bland garnish on an otherwise good steak.
However, I also agree that they've exhausted this current framework for the Horizon series. If Forza Horizon 5 is more of this, just with another Big New Feature (TM) or two, I will be disappointed. It's time to evolve the series, and take some new chances. A new console generation is the perfect time to do that. I also think Microsoft need to re-think this bi-annual "tick-tock" release cycle with the Forza games. Two years just isn't enough time to do much more than slowly iterate on your previous work, and make hard choices about which things you can spend time working on, and which you can't. The paint/livery tools and the auction house, for example, clearly weren't really touched at all for FH4. These games need to move to a three year cycle, at least. And six Forza-branded titles in a single console generation, is approaching over-saturation. Absence makes the heart grow fonder, and all that.
For now, I just want them to stop wasting their time on new "Horizon Stories" in the monthly updates, and put that time into focusing on more content for the game's strengths, and fixing it's more important weaknesses.