If I throw a four seam fastball with a softball or baseball , the ball will be much straighter than if I would throw a 2 seam fastball, so I have to disagree somewhat, however when throwing a curve ball you would actually want all 4 seams catching the air to make it curve more, seems a little confusing, but true. I love discussing these issues, everyone has different opinions, and it is cool hearing them.
There are some factors here you may or may not be aware of in regards to how the pitcher holds the ball and how she actually releases it. We don't know each other so don't be upset if this seems trivial. I'm simply trying to explain myself and possibly help everyone understand a few things about pitching and the results you'll normally get when done properly.
If you hold a softball to throw a fastball and properly follow thru with a finish wrist snap that resembles a parent giving a small child a tap on the butt (spank), the way you position the ball in your hand will determine how many seams will tumble in its path to target. The fastball will spin as it comes off your fingertips. That spinning motion will have the ball spinning downward; allowing us to argue that there is no such thing as a fastball in fastpitch softball.
That downward spin is the same as a drop ball so in essence, it is a "form" of a drop she is throwing. This is why some pitchers throw a sinking fastball that has a downward trajectory. This is mirrored by a baseball pitcher's tailing fastball. The key factor is how the ball is held and how it tumbles at release. 4 seams will effect the ball more than 2 seams.
If your pitcher throws a fastball where at release she rolls her wrist to spin the ball sideways as if shot out of a rifled gun barrel, the ball WILL travel in a straighter line to target. Just as the rifling in a barrel spins a bullet on a straighter path, the ball will do the same. Most of the pitchers that throw this fastball do not reach full velocity because part of the wrist snap's energy is used to spin the ball in a direction other than towards the target. We use the term "spank" to relate to our wrist snap and encourage a cleaner release and more wrist snap. We call it "putting the spank in our fastball."
Having said this, if a pitcher holds a ball to throw a 2-seam fastball, and instead throws an executed curveball, the ball will be spinning with 4 seams cutting the air. If she holds the ball for a 2-seam fastball and executes the fastball motion with a sideways rolling release, this ball will travel straight/flat to target as the 4 seams will be spinning like out of a high dollar Weatherby rifle.
My point is; without having a pitcher demonstrate each pitch so we can compare notes about how she executes each one, I could be or all of us could be missing some key elements that give us the true reason on "why" we disagree on how a pitch reacts when thrown. It's easy to miss some of the details when our pitchers throw with some outstanding motions, arm speed, and wrist snap.
I understand how easy it is for us to disagree, but let me finish with one last thought. I'm not a stick in the mud instructor that expects a child to throw with a patented motion. We're working with human beings here and they are all unique in their own way. If I have a girl that can throw her riseball as a rising curve and she struggles to get it clean, I'll go with developing her rising curve. My objective is to get the ball to move. If everyone threw the same curveball and it reacted the same for everyone, we'd never throw that pitch because hitters would learn to hit it. It's the human element that gives each pitcher "personality" which in turn becomes a style.
I'm fine with that!!!