The four-seam fastball is a pitch that is used often to get ahead in the count or when you need to throw a strike. The type of fastball is intended to have minimal lateral movement. Please see picture for detailed illustration (or visit our pitching library to view the full picture) of where your fingers should go. There are two general ways to throw a four-seam fastball. I will try to explain each of them and again, the picture speaks a thousand words.
The first and most traditional way is to find the horseshoe seam area, or the area where the seams are the farthest apart. Keeping those seams parallel to your body, place the index and middle fingers perpendicular to them with the pads on the farthest seam from you. The thumb will then rest underneath the ball about in the middle of the two fingers. With this grip, the thumb will generally have no seam to rest on.
The second way to throw a four-seam fastball is to take a baseball and find the area where the seams are closest together. Arrange the ball so that those seams are parallel to your body. Place your index and middle fingers on the baseball perpendicular to the seams. What you may find here is that the younger athletes will feel more comfortable with those two fingers apart, as they will have more control because of smaller hands. What you generally want to teach is that those fingers need to be somewhat together. The pads of those two fingers need to rest on the seam that is farthest away from you. Try to keep equal pressure with those two fingers. If you increase pressure in one and decrease the other you may have some movement, but very little. The thumb will then rest underneath the ball about in the middle of the two fingers. I tell my pitchers (assume a right-hander here) that they need to rotate the ball counter-clockwise just enough so that the thumb will fall on the apex of the horseshoe part of the seam. This gives some pitchers a little better control physically, if not mentally. A critical thing here is that the thumb needs to rest somewhere on the side to middle of it's pad. You don't want to push the baseball all the way back into your hand. That will decrease both control and velocity of the baseball.