Uber's San Francisco office sent a guide to all of its drivers in 2014 that explained how the driver-rating system works, and how drivers can improve their scores.
The document says that 4.6 is the important number when it comes to driver ratings. If a driver's rating is 4.6 or lower then Uber is going to start considering kicking that driver off the system.
This chart shows the distribution between the different driver ratings: Uber
The 2014 document says that only 2-3% of drivers a