How does the series Lucifer end or how to find out?

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.

Netflix has revived Lucifer for its fourth season

The streaming giant has picked up the fantasy drama for a fourth season. Lucifer is getting a second life.

Netflix is reviving the canceled Fox drama for a fourth season. The DC Entertainment series, which hails from Warner Bros. Television, was axed in May. An episode count for the new season has yet to be determined.

The new season is expected to be half the number of episodes as a "typical" Lucifer series but at least the story will continue.

I know the question was asked under the conditions that Lucifer was being cancelled, but the current update is that Lucifer ran for 6 seasons and has formally been wrapped up with its final season on 10th September 2021.

It ends with

Lucifer : finds his calling; helping redeemable souls in hell
Chloe : lives on Earth till her death and reunites with Lucifer in afterlife
Dan : in heaven with Charlotte Richards for eternity
Amenadiel : becomes the new God
Maze : marries Eve, and continues bounty hunting with her
Trixie : probably alive
Rory : returns to her original timeline
Ella : last shown being in a relationship with Carol (Dan's ex-colleague)
Linda : last shown raising Charlie on Earth