Hi LadyW,

The longer/shorter span differences you mentioned do react in that manner as stated by your proshop.  However, in my opinion and experience, the span difference would have to be greater then 1/8.  That number normally would not produce a significant difference for most people, assuming the 4 inch is at a relaxed grip for you.  In your particular case, the roll is probably going to be more useful and effective because shortening the span will not automatically produce revolutions, you would still have to be able to flip your wrist thru the ball harder and faster then normal to create more revs.  The shorter span (relaxed grip) simply allows a person to rotate their hand faster around a ball vs a longer to stretched span.

Due to your slower ball speed, it's more important for you to find equipment that will help you get down the lane easier or more naturally without added muscle effort.  A higher rg ball tends to promote this type of reaction.  

Most important of all things these days is to match your game, ball, and lane condition up to your advantage.  Try not to force something to happen as we have all seen that fighting a lane condition becomes more difficult and we tend to score lower then when we play(or feed) the lane and take what's given to us.
Hope this helps....
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Rick Leong
Ten Pins Pro Shop
Track Pro Shop Staff
Vise Grip Staff