Conversation so far:
what were the orgins of this song?
McCartney, Harrison and Starr originally intended to record some incidental background music, as a trio, for the Anthology project, but later realised.
What did they realized?
according to Starr, that they wanted to record "new music".
What kind of sound is "free as a bird" inspired by?
CANNOTANSWER.
why is it called free as a bird?
CANNOTANSWER.
who was involved with the song orginally?
"Free as a Bird" was recorded by Lennon.
anything else interesting about this song?
McCartney then asked Ono if she had any unreleased recordings by Lennon, so she sent him cassette tapes of four songs.

Question: what were the four songs?
Self-contained rewrite: what were the four songs that was unreleased recordings by Lennon?