Personally, I don't think there are enough improvements to warrant a purchase if you have Gekko Ninja Force 3. The good changes are the new tag mode, new combo-jutsu meals, and improved replacement-jutsu. And the characters I suppose. The bad changes are the character select and mission mode, which YOU HAVE TO PLAY. If you haven't noticed by now, I hate mission mode. Even if it didn't require any real talent, unlocking stuff in 3 was actually fun, as opposed to in 4 where I have to complete every mission because I can't read japanese (which I think isn't even a real language anyway, but more on that later). I suppose if you were starved for a little bit more Narutos it would be worth it for the missions and story mode and couple extra fighting modes, but it still feels too much like the same game for me to want to shell out 60-70 bux for it.
As for spoilers, I find it easier just to ignore the anime all together