Fourth gear is in the transmission. It is described by a ratio. If you have a fourth gear ratio of 1 to 1, that means the fourth gear in the transmission turns 1 time for every 1 turn of the engine.
Pictured above are manual transmission gears.
What you do:
Choose a gear ratio that gives you the acceleration and speed you want.
Factors to consider:
The HiPerMath equation is:
miles per hour =                           rpm x tire diameter
transmission gear ratio x rear end gear ratio x 336
Try out the gearing on the track to see what your car will do.
mph = miles per hour
rpm = revolutions per minute