Downloading a "Perfect" save file from sites like GameFAQs provides instant access to Master Mode rewards, all shop items, and difficult-to-unlock characters like the 4th Hokage (Minato Namikaze) . How to Unlock Characters Manually
For those playing on PC, importing save files requires a few specific steps: naruto ultimate ninja 4 ps2 save data
If you prefer to earn your way through, specific modes unlock different tiers of the roster: Downloading a "Perfect" save file from sites like