Fastball
A four-seam fastball is a variant of the straight fastball. The four-seam fastball is a pitch that is used often by the pitcher to get ahead in the count or when he needs to throw a strike. This type of fastball is intended to have minimal lateral movement.
There are two general ways to throw a four-seam fastball.
With this grip, the pitcher will feel more comfortable with those two fingers apart, as they will have more control because of smaller hands. The pads of those two fingers need to rest on the seam that is farthest away from the body, keeping equal pressure with those two fingers. The thumb will then rest underneath the ball about in the middle of the two fingers. This gives some pitchers a little better control physically, if not mentally.
Critically, the thumb needs to rest somewhere on the side to middle of its pad. It's not necessary to push the baseball all the way back into the hand, which will decrease both control and velocity of the baseball.