Ah...the Mount Rushmore thread...it may be "better" to break up the artists by era...Gold, Silver, Bronze, Copper, Modern, etc. However, if you're only allowing the top four most important artists as opposed to favorite artist, then subjectivity must be set aside? Are we strictly speaking American comic books? Would an artist/writer carry more weight than just an artist alone?
My personal favorites:
Baker
Everett
FIne
Schomburg
*Frazetta if we consider him to be a comic book artist rather than paperback/magazines paintings/drawings