Yeah, I used to think that, too. I ended up in an argument with a guy much more intelligent than me in that science stuff... like Rick did. I concede that now...
The relationship and effect that drag has on influencing the MOVEMENT of a ball is not a direct relationship (4 seam does not equal twice a 2 seam), as you suggest. You'd think a smooth ball would NOT break as much as a ball with rough surface, but this has been refuted by the old dimpled golf ball, having less drag than a smooth one.
Furthermore, think of those seems cutting with taped balls... how many pitchers really get perfect 12-6, 6-12, etc... spin? It's the rate of spin and speed of the pitch, as well as the the pressure and turbulence around a ball (the boundary layer) - that ultimately influence the pitch the most... in regards to the effect of spin... There are also many environmental and lunar effects that do their own damage, too...
I'm not saying that 4 seams can't influence more than 2 seams... but the relationship to movement is not 2:1 because of a grip - and seldom does a pitch actually utilize the 4 seams equally (softballs aren't symmetric either)... and as such, the center of pressure changes, allowing for variations in every pitch.
I may be wrong, as usual... but this is how I understand it... I'm definitely open to ideas/info on this one...
Expecting 2:1 movement with 4 vs 2 seams is unrealistic because the ball is much bigger than the seems and to suggest that wouldn't make sense. I know that Rick mentioned a Professors' belief that the number of seams doesn't matter. Professor's make mistakes all of the time. Research is refuted more often than we are led to believe. But I will offer something up. I know from personal experience that larger seams make the ball move more than flat or no seams. In my logical world it tells me that the seams have a direct correlation to how much the ball moves. It would stand to reason that having 4 seams turning would add a little more bite to the ball than just two, if bigger seams matter as well.