Mike,
Put another way, the aspect ratio (W/H) of the sensor has NOTHING TO DO with the number of pixels in each direction. A 4/3 sensor could be composed of ANY number of pixels as long as the ratio of the number in the width divided by the number in the height is 4/3. The following are not actual pixel counts, but are theoretical sizes, all of which would be 4/3:
Could be 4000 x 3000 = 12Mp.
Could be 1200 x 900 = 1.1MP
Could be 6000 x 4500 = 27Mp
Brad