Though offers can generally be freely revoked, what are the 4 exceptions?

Are pre-existing duties valid consideration? Under battle of the forms, what happens to additional terms in an acceptance between two merchants? Under battle of the forms, what happens to different terms in an acceptance between two merchants? What are consideration substitutes? What are illusory promises and how do they affect a contract? What are requirement and output contracts? What are the methods of terminating an offer? What are the UCC Gap Fillers? What is an option contract? What is promissory estoppel? What is the effect of a conditional acceptance on an offer? What is the effect of an offeree beginning to perform in response to an offer? What is the Mailbox Rule and when does it apply? When are advertisements valid offers? When is past or moral consideration valid?