Man are you going to get yelled at for not searching. You know you could look behind the drivers side visor or the owners manual for help. But since I am a nice guy:

You can shift to 4H under 20 MPH or while not moving either is fine. It is best to shift while on level ground or at least somewhat level in order for the hubs to lock. Once the hubs are locked they stay locked until you move in the opposite direction in 2H. But as long as they are locked, you can shift into 4H at any speed. For 4L, you must stop, put the truck in neutral, then shift. Do the same in reverse to go back to 2H or 4H.