Generally the four-seam fastball is a straighter pitch with a 1-2 mph increase in velocity over the two-seam. When throwing opposite arm-side, righty away from righty or lefty away from lefty, you have the option to throw the four-seam straight fastball to avoid the ball moving back over the middle of the plate. The four-seam fastball opposite of arm-side is a good lock in pitch early in the count.
To execute a lock in pitch, you must have good extension and a good release point to get the ball to the other side of the plate. Once you find that release point and are “locked in”, you can begin to throw your other pitches in relation to the four-seam fastball release point with extension.
It is important to note that it can be very beneficial to be able to throw the two seam fastball in Zones 1-6. The 4 seam opposite arm side will help keep the ball straight if you WANT it to stay straighter. If you would rather have more movement then it would be a good idea to practice the 2 seam in all those Zones.
The 4 seam fastball is generally used when you need to elevate the ball to the top of the strike-zone (Freeze Zones) or above the strike-zone (Purpose Zones, Red Box). Some pitchers prefer to use the 2 seam fastball when throwing the arm-side Purpose Pitch so the movement of the ball helps get the pitch inside. But others have found that the 2 seam can move too far inside and hit the batter. Experiment with the 2 and 4 seam fastball in all Zones so you know what you do best.