4'33" actually has two main points. The first is clear: any sound can be musical. The second is more subtle: a composer does not need to specify which sounds will be heard in his composition. This runs against Varèse's definition of music as "organized sounds", which Cage endorsed in his earlier music, since there is no organization beyond what nature provides.
Parenthetically, Cage was friendly with Robert Rauschenberg and he was familliar with his white painting... a visual art equivalent to his 4"33'?