There's a month or two time skip between the end of S3 and the beginning of S4 and apparently in the meantime Chloe and Lucifer had not interacted at all.
Each character will deal with repercussions from S3…like for instance, Lucifer himself in this season will be a lot darker, Eve (a new, recurring character) will try to bring forth his old, darker side. And there's this issue with his devil face and why did it return at that very specific time (because at the end of S3, Lucifer didn't know his devil face had returned) – is it because he killed a man? Is it because he liked it? At some point in 4x05 also gets shoot protecting Chloe and gets taken to the hospital…
Chloe now must face the truth that her roommate is a demon and that Morningstar really is the devil. She has a sense of responsibility as a mother and thus keeps her distance from both Lucifer and Maze. She decides to turn to a new character, Father Kinley, who has his own dark plans towards Lucifer and rumour has it, he's trying to manipulate Chloe against him.
Father Kinley is kind of obsessed with this prophecy having to do with the Devil walking the earth and finding his first love. And when that happens evil will be released upon the world. He truly believes that.
Dan blames Lucifer because Charlotte Richards is dead. He's really going to be wrestling with his demons. Because, of course, he’ll have a ton of anger in the wake of Charlotte’s death. He’ll find himself wondering if he’s just been playing ‘the good cop’ and that maybe Pierce was right about him all along when he said, 'You’re the corrupt cop'
We're also being introduced to a completely new Ella, apparently darker, more 'sex & rock'nroll type (as quoted by the actress) and she's going to continue questioning her religious beliefs. She starts spiralling because she's lost her rock in God. Although we will never meet new members of her family, we'll see more of her - physically and emotionally - this season. Also (lol) there's an episode where Lucifer and Ella go to a nudist camp and we see her naked. The actress happily pointed that out.
Linda is going to have Amenadiel's baby and it is rumoured that Lucifer and Amenadiel's sister Remiel (played by Vinessa Vidotto, who is set to appear in a few episodes) will try to kill the nephilim baby. The last two episodes are according to bts photos extremely action filled with a lot of cgi usage.