Your pcm is programmed to shift high enough that you won't reach the 4th gear when the throttle is 100%, which you would be at the track.
As for modding w/o tuning. Our pcm's are setup to work in climates from the heat of the equator to the extreme cold at the north/south poles, in all humidty ranges and altitudes (varying levels of oxygen to burn). This gives them the ability to function fairly well with basic mods. A tune however can remove some restriction and dial things in for the conditions you are operating within, this yields more power.
