The grip is across the seams, as seen in the photo above. It is thrown with regular mechanics and velocity.
The four seam fastball is the straightest fastest pitch a pitcher has to offer. I have found that even the best hitters have a difficult time catching up to this pitch when set up with a change up, when thrown up in the zone. The reason the pitch is effective is, gravity pushes the ball in a downward motion but the back spin will give the ball lift which results in a very straight trajectory from the pitchers hand to the catchers glove.
This pitch is most effective when thrown high out of the strike zone. A batter who carries their hands low when addressing the plate will not catch up to this pitch. This pitch is know as "High Heat."