Get 99 lives
This glitch can be triggered while fighting the boss of stage 3 (Porky Pig's stage). Make sure you're on your last life. Get both your health and the boss's health down to one, then wait until it shoots its bullets. Defeat it, then quickly fly into a bullet and die. The Stage Clear screen will appear before Porky reaches the bottom of the screen, and you'll progress to Stage 4 like normal.