Four years into the mission, the understanding of the Fermi LAT detectors properties and data analysis is now far beyond the required initial specifications. Thanks to a careful analysis of flight data, we were able to trace back some of our major sources of systematic uncertainties to the use of slightly incorrect detectors calibration constants. In this poster, we report a major collaboration effort to update the LAT detectors calibration constants, to use these to reprocess the first 4 years of raw data, and the improvements observed on the low and high level analysis. The Pass7 reprocessed data (also known as Pass7 P202 data) are still being validated by the collaboration and should be made public by the end of the year 2012.