Jab (Shoutbox "One")
Jab / Cross (Shoutbox "Two")
Jab / Cross / Hook (ShoutBox "Three")
Jab / Cross / Hook / Cross (Shoutbox "Four")
This video is actually about a five punch combination, but does an excellent job of illustrating good punching form. The Shoutbox "Four" is this combo, minus the last hook punch.