The general consensus is that a four-seam fastball is thrown with less movement and more velocity than a two-seamer. Check the San Diego Padres rotation, using 2010 PITCHf/x data from FanGraphs:

Pitcher Four-seam MPH Two-seam MPH
Garland 88.8 89.4
Latos 93.5 92.8
LeBlanc 86.4 82.8
Correia 89.9 89.5
Richard 91.4 91.3

Yeah, check that rotation. Prior to looking up the data, my theory was that the four non-Clayton Richard (since we’ve already looked into the data on him) Padres starters would have four-seam fastballs that registered a few MPH faster than their two-seamers. Instead, we see not only Richard averaging the same velocity with each pitch, but also Garland (a faster two-seamer, in fact), Correia, and Latos (within one MPH). LeBlanc, the only one who has a clear difference between the two pitches, uses his two-seamer sparingly (less than 2% of the time).

What gives? Well, we have (at least) three possible explanations:

  1. The Padres employ a staff that just so happens to throw both of their fastballs (ignoring the cutter, for now) at similar speeds. Perhaps they just happen to be on the same staff together or maybe it has something to do with pitching coach Darren Balsley or the Padres organization in general.
  2. We are again encountering problems in classification, by Gameday, and are not witnessing real differences in the two pitches, rather difficulties in properly classifying them.
  3. I’m crazy, and four-seamers and two-seamers are generally thrown at the same speed.

I think we can rule out choice three, as Dave Allen says here that four-seamers are indeed thrown faster — “about 1.5 mph faster than two-seam fastballs and 3.5 mph faster than cutters.”

My guess is that it is some combination between one and two. But, frankly, I have no idea what the correct answer is and whether or not it is significant. Any thoughts?