Defensive linemen are usually in the center of the action and often face blockers coming at them from a variety of angles and directions. It is vital that a defensive lineman assume a stance that allows him to move forward and react to both his right and left. The crucial thing that you want to avoid is standing straight up and making your entire body open to the offensive blockers.
To achieve a good four-point stance get into the breakdown position with your body, feet, knees, head and eyes facing straight ahead.
From the beginning breakdown position, reach straight forward, placing both hands on the ground slightly in front of his shoulder pads.
Make sure you are on the balls of your feet. Your hips and shoulders should be even, with your back level. Defensive linemen that line up with their hips lower than their shoulders will often have their first move straight up opening them up to the offensive blocker.
It is most important to keep your head up and eyes scanning the offensive players in front of you. Seeing the ball, or having an offensive lineman move, should be the trigger for you start your charge.
From this stance you should be able to charge forward low and hard using short, fast, power steps and still be under control and in position to quickly react to any blocker attacking from the right or left. Also, from a four-point stance a defensive lineman can be in position to play with his hands above their pads and rush the passer on an offensive pass play.