Extra accuracy points
To get more extra accuracy points, shoot at the "End of Level" sign at the end of each level.
Level Skip on Level 4
After completing level 4, rapidly press the START button on the gun (it's the very bottom button on the front hand grip.) As soon as the screen reads LEVEL 4, rapidly press START to skip ahead 2-4 levels. If your timing is good enough you can skip all the way to the final level.