I'm not real familiar with the 4R55E, but here's what I can say.
Your 4R55E has speed sensors on the input and output shafts of the transmission. The computer "knows" what the gear ratio of each gear should be, so it know what the relative readings of those two sensors should be. It sets that code when it gets the "wrong" reading from one of those two sensors.
1st question I would ask: Is it actually shifting into 4th gear or not? If it is going into 4th gear as it should, then the problem is likely in the sensor circuits. If the computer is commanding it to shift into 4th gear, but the transmission isn't responding, then there's some condition (such as a maladjusted or broken band as suggested by BrooklynBay) in the transmission that is preventing it from responding to the computer's commands.