Technology

Pretty much just incremental improvements. Faster processor, more RAM, slightly better peripherals (like faster ethernet or newer wifi specs), but essentially they're all the same thing.

It simply may be overkill for some projects. No reason to get a 4 when a 3b or pico will do the job just fine at a fraction of the cost. Same deal with the 5; It’s more powerful, but it’s also more expensive and not every project needs the most recent version. The original goal of the Pi was to provide cheap accessible processing for whatever you may need. But as innovations keep happening, things keep getting more expensive.

For an extreme example, you wouldn’t build a $3500 gaming PC for your grandparents who will only use it for Solitaire and checking their email.

It's not bad, just more expensive and more powerful, so it would be overkill for certain projects. If you want a cheaper or smaller (but less powerful) option, you'd have to get it from somewhere else.

If you want to use it for emulating games for example, you'd probably want the best performance you can get. If you're just making a digital picture frame though, you don't need to spend the extra money for a Pi 4 and can get away with the cheaper models.

I'd recommend looking up what you want to do and seeing what model Pi other people have used. I use a Pi 3 to control my 3D printer, which isn't too demanding of a task for it.

Depends. Most of my Pi’s are 3 or under, and they’re doing mundane tasks like printserver or VPN. This takes little power or speed. I did use a pi B+ with Kodi and that tended to choke up occasionally, I think mostly due to heat though even after applying heat sinks. The 4 and 5 should handle that job a lot better.