August 2014:

Katy says she wants to do an acoustic album and a "full dance album" in the future.

November/December 2015:

Katy is in the studio with Greg Wells (she has said before that he is always the first person she works with when starting a new album)

May 2016:

Katy confirms that she has started working on the album

A Twitter troll prank calls Katy while she is in the studio; a new song that later turned out to be "Rise" can be heard in the background

Katy's Twitter account is hacked: an unfinished new song called "Witness" is leaked as well of a snippet of "Rise"

July 2016:

Katy releases "Rise" as a stand-alone single and says she is "taking her time" on the album. It was produced by Max Martin and Ali Payami and co-written with Savan Kotecha, indicating that they may have worked on the album too

August 2016:

Swae Lee (Rae Sremmurd and "Formation" co-writer) says he has written a song for Katy that is "pretty dope"

Katy says she is "experimenting and taking chances", and that she is "trying a lot of different producers, collaborators, and styles"

September 2016:

September 6: she uses the fish emoji for the first time, saying she "caught one"

September 9: she posts a selfie on Twitter while in the studio

Sarah Hudson posts a short video of Katy dancing with an urban-sounding song playing in the background. The song doesn't produce any Shazam results, causing rumours that it is from KP4 - possibly the song that Swae Lee wrote for her

DJ Locksmith (Rudimental) says he is going to the studio with Katy in December.

Katy posts videos in the studio of Ferras dancing to several 90's dance/deep house songs and she says that these songs are her "muse", indicating that we are getting Walking on Air 2.0 (couldn't find receipts for this)

September 14: Katy & Jack Garratt tweet each other about her new music, indicating they are working together

September 16: fish emoji

September 20: fish emoji

October 2016:

October 4: She says on Twitter that she is attending meetings, possibly related to KP4

October 5: fish emoji

October 8: fish emoji

Katy is in the studio with Max Martin and Shellback

Katy posts video of her working with Kevin Garrett (who co-wrote Beyonce's "Pray You Catch Me") (couldn't find receipts for this either)

Chance the Rapper tweets "Can I get a witness" and likes a tweet about collaborating with Katy, pretty much confirming that he will be featured on the song

October 18: fish emoji

November 2016

November 3: fish emoji

Trump wins the election - Katy has a Twitter meltdown and turns her Twitter page white for the rest of the year

November 16: Katy posts an Instagram picture with a Toni Morrison quote about how artists must "go to work" following Trump's election and that "there is no time for despair"

November 19: Katy uses the fish emoji, saying she "caught a big one this week"; this song may have been inspired by Trump's victory

December 2016:

Katy posts 2 new snippets on Instagram . One of the people in the video appears to be Shellback, indicating that they may have worked together on multiple songs