what would a bad map sensor do to a 02 saturn sl five speed

Your engine’s Manifold Absolute Pressure (MAP) sensor could be the culprit if you're still getting lousy gas mileage after a tune up. A MAP sensor continually compares atmospheric absolute pressure to the intake manifold vacuum and sends the appropriate voltage signal to the vehicle’s computer. The computer then decides whether to increase or decrease fuel supply to the engine and adjust spark advancement accordingly. A bad MAP sensor sending the wrong signal to the computer can disrupt fuel consumption and overall engine performance.

it will not go above 4000 rpm in neutral? or is this when accelerating?

4k is the standing rev limiter under 5mph. Sometime if youre getting a p0341("cam sensor") code, it wont rev past 4 grand.