From the current information, I believe there was nothing really written yet for the cancelled season 4, since initially whole series was supposed to finish at episode 18 in season 3
On Twitter, Lucifer executive producer Joe Henderson indicated that the series will end with a closure in Episode 18. ”We designed this season to be an 18 episode arc because we originally were going to be 18 episodes. But then we got 4 more! Surprise!,” he wrote. “So we decided to create four episodes that let us play around a bit. Dig deeper into the character stories, play with format a bit, etc.” The four episodes “stand on their own, but also pick up plots we’ve introduced and bring in new stuff we’ll play with season 3 source
Still, it seems that Henderson is disappointed in the cancellation and hopes that the show will come back:
We created a season finale with a huge cliffhanger so that there was no way Fox could cancel us. Instead, we're going to frustrate the hell out of you fans. I'm so sorry for that. By the way, the Lucifer finale is the best hour of television I've been lucky to write. It wraps up everything from the season. But what it teases for season 4... well, maybe, just maybe, we can #SaveLucifer and show you. source
In either way I wouldn't expect for the supposed whole series finale hints to surface any time soon - either they never existed or they do, but their leakage would damage any chances this show would have in resurfacing.