Well, it’s always multiplayer, meaning you will see people out in the world constantly. A lot of content (like dungeons) is instanced to your party, though, so you won’t see randoms where it really matters. Personally, I have only played solo so far except for the aforementioned randoms :). I’m sure it would be very fun with others, though!
Personally, I like it way more than 3. I could never get into the older games (including 3). Others say it’s a “reskin of 3” which I think is kind of ridiculous.
The story so far is actually pretty intriguing. I mean, it’s not mind-blowing, I’m pretty sure of what will happen, but it’s still good for a game in this genre.
Replayability is really endless, but, I don’t know if I’d say it has more replayability than the older games, it just may be more replayable for you if you like it more.