I played a little bit of 3 but that's it. No reason not to start with 4 from what I've seen.
You can do the whole game solo if you want though it's definitely more fun with other people. The game is always online even if you're playing solo so keep that in mind... you will always need an internet connection and you will see other real players running around in the world, killing enemies and whatnot. Haven't got to endgame yet but I'm sure you'll need groups for all of that stuff.
The game is crossplay which is great, I'm on PC but I've got friends on PS5 and Xbox I've been playing with. Both consoles have full Discord support now so we can all voice chat easily. I'd be down to play with anyone here if they needed help, schedule permitting.