Now did they do the four actors playing one character thing because of Heath Ledger's death, or was that the intention from the very beginning?
It was an emergency response to his death.
Farrell was my least favorite of the replacements, but I think that in many ways he
was playing a different character than Ledger did... Tony [and everyone else] was a character in a story he was telling about himself, projecting an idealized version for everyone else and maybe even kidding himself about his true nature. [insert Roger Clemens reference here

] One of the things Parnassus and the mirror do is take aspects of your inner self and create a world from them. Tony is more-repressed than most perhaps and has multiple levels of deception, thus it take three trips to expose all of the parts of him.
Farrell's Tony probably
is the "real" one, sadly.