4th-D Slip is Ninten's Side Special move in SSBTUF. Ninten runs really fast forwards, and you can also change the direction of it. You can continue running as long as you hold the special move button. Ninten is invincible during this but he can't attack during this and if he runs for too much time he will start to damage himself.
4th-D Slip is a PSI technique that appears in Mother. It is learned by Ninten. Using this, Ninten and friends can escape from battle with a 100% chance of escaping, while escaping normally would have a chance of failing.