i'm so exited for the Katy feat. Rih IT WILL THE BEST SONG EVER
There is no song with Rihanna. It was NEVER confirmed. Don't believe anything coming from twitter UNLESS it's coming from Capitol's twitter and/or Katy.
Katy and Rihanna have both confirmed, multiple times, that a song by the both of them is coming though.
Not to say that it will definitely be on this next album, but there's a pretty good chance of it.
Though I'd personally rather it didn't happen, to be honest. I can't really imagine a Katy and Rihanna song being good..