I know at the time of the Godfather Brando was very established at the time as an great actor. I know Pacino and Keaton only did 2 movies before staring in the Godfather, Caan had done about a dozen before and Duvall was in To Kill a Mockingird and the MASH movie along with more than a dozen others.

My question: What was these legends' status before the Godfather as actors (Caan, Duvall, Keaton and Pacino)?

Speaking from personal knowledge at the time (that "The Godfather" was released):

I knew of James Caan from his role in the TV movie "Brian's Song" and I thought he was an OK actor, but only "OK".

Duvall was the most familiar to me (of those listed above). I had seen him in a few movies, and absolutely loved his portrayal in "M*A*S*H".

I hadn't seen Keaton or Pacino in anything before I saw them in "The Godfather".

You'll find useful info on this subject in Harlan Lebo's "The Godfather Legacy" (a must for any serious player on these boards). Briefly: only Duvall had been around long enough to establish a tangible reputation. FFC chose him and Caan because he'd worked with them previously on "The Rain People." Keaton had done theater, TV commercials and was a member of Johnny Carson's "stock company." Pacino was up-and-coming, but still relatively unknown.
One thing that's not said explicitly, but which I suspect: FFC may have chosen them in part because they were relatively unknown, and he could therefore get them cheap. One of the reasons Paramount chose FFC to direct GF was that he was young and relatively unknown, and they thought he might be able to bring GF in cheap(!!!) He wanted Brando and knew he'd have to pay big bucks for him, so perhaps he chose the others in part because they didn't command high salaries. As you probably know, Pacino got all of $50k for his work in GF.

This is what comes to mine:

Caan did to a lot a bit parts in films. I think his notable ones in The Rain People (FFC film), and Brain's song. I think he worked a lot with Duval in Roger Corman flicks.

Keaton had a bit part in 1970 movie Friends and Other Strangers and NBC's Night Gallery in which she played a nurse, she also did HAIR on broadway.

Pacino was more noted in off-broadway theather and he won an Obie for Does the tiger wears a Neck tie. He had a guess starring role in ABC's cop show NYPD and he met his old girlfriend Jill Clayburg during filming.

Duvall did some teleplays in the 50's, Roger Corman films, and of course MASH and Mockingbird.

I feel like Bonasera in Don Corleones study questioning Turnbulls post, but I will give it a shot

From what I understand at that time, wasn't Brando's career more or less on the downside? Didn't he have to somewhat audition for the part, with the famous shoe polish and kleenex story? Im not saying he came cheap, but cheaper then other names that were bandied about, such as Anthony Quinn. Of course after the success of Godfather, Brando started acting like Brando, demanding the high salaries again, such as a high one for about 12 minutes in Superman.

Thanks for the info, that's what the states generally indicates and what my impressions were. I have another question. Is it true that the studio wanted Martin Sheen to be Michael? What would that have been like?

They screen tested Martin Sheen, it's on the Extras DVD, but he didn't have "it." He was damn good in Apocalypse Now, but he wasn't playing a Sicilian in that film, lol. They also really wanted Robert Redford, but FFC kept squeezing Al in every chance he got. The studio wasn't gonna use Al until Brando stood up for Francis. Brando had such a presence, both on screen and off, that no one really questioned him.