I'm finishing at the top end of 4th gear with 3:55 gears and 26" tires I'm shifting with the shift light set at 5500. I'm thinking that I might do better with 3:27 or 3:08 gears. I like to see where others are running and what gear are they finishing in.
Some taller tires might be a better investment than different gears. Keep the 26" tires for street use and have some very sticky taller tires for the track. The soft rubber won't last long with street driving, but will do wonders for your track times.