Not my personal experience, but I understand that 4-speeds are sometimes run for one or more of the following reasons:

1) reduce weight
2) reduce rotating inertia
3) reduce friction
4) reduce cost or complexity
5) take advantage of shift-linkage geometry for 3-4 which is often less awkward than 4-5.

For example, I've heard that some ITC VWs run the 4-speed with a taller R&P instead of the 5-speed with a shorter one and find it quicker at both NHIS and/or Lime Rock. In that particular case, I believe that the 5-speed has the same ratios as the 4, except for a closer 5th tacked on (and an optionally shorter 3rd for the 4-speed).

Keep in mind that we really only use two gears at most tracks, most of the time. So some would say "why lug around the extras?"!