There are two types of RPM (speed) sensors found on StarTrac 4000 treadmills. There is an "optical" RPM sensor and there is a "Hall Effect/magnetic" or commonly referred to as "cherry" sensor.
On the StarTrac 4000 treadmills, the "optical" sensor reads a plastic disc and the "magnetic" or "cherry" sensor reads a metal disc. One cannot be used for the other.
The "optical" RPM sensor is designed so that the plastic disc sits centered between the fingers of the RPM sensor. As the plastic disc spins, the RPM sensor is picking up the gaps in the plastic disc which is interpreted by the electronics as the speed of the drive motor.
The "magnetic" or "cherry" sensor is designed so that it sits in close proximity to the metal disc. As the metal disc spins, the magnet is detecting the gaps in the metal disc which is interpreted by the electronics as the speed of the drive motor.
Certain versions of the StarTrac 4000 treadmills equipped with a "magnetic" sensor used a design that is physically similar to the "optical" sensor used today. These were a small circuit board design with the sensor mounted to the circuit board. This version of the "magnetic" sensor is no longer available and has been replaced with a newer design.
When looking for a replacement speed sensor for the 4000 StarTrac treadmills, it is best to identify the type of disc used in conjunction with the sensor: plastic vs. metal. Knowing this will guarantee the proper choice if there is any doubt.