Your topic has prompted me to look into it. This must be the reason your getting more noise using a 4/3 sensor.
Compared to a larger sensor with equivalent pixel count, a Four Thirds sensor gathers disproportionately less light per pixel. Not only are the individual photosites smaller, but each loses more of its total area to support circuitry and edge shading than a larger photosite would. With less captured light to work with, each photosite requires additional amplification, with associated higher noise as well as reduced dynamic range. A telecentric lens design can mitigate this problem, but the sensor remains more sensitive to the angle of incoming light, and has more pronounced image corner light falloff.
