Unfortunately, the $100 price tag puts the Pi 4 in an awkward place: Not cheap enough to be a hobbyist board, not powerful enough for "serious" things. To me, that's extremely unfortunate, because I think many, many more people need a cheap, underpowered board they can tinker with than need a PC with some exposed GPIOs.
It's $75 for an RPi 4b with 8GB of RAM! And from my experience it's quite powerful enough for a lot of "serious things", including most casual desktop computing needs. And it's only $35 with 2GB of RAM, which is all you need for all sorts of headless server tasks.
It's worth noting there's also some solid competition for Raspberry Pis by other companies making similar hobbyist boards. I'm pretty sure there's one out there that might fit your needs but I forget the name of it.
I heard that things like the Banana Pi (or similar) use horrid chipsets that have very limited support, so they might not actually be of similar utility. If you know of any good ones, though, I'd be grateful!
Nowadays I use an ESP8266 for most things, but it'd be nice to have a small, reliable Linux machine for things like file servers at my parents' house.
