Since the original thread in 2014, there has been more information uncovered on the 4596. A Lew Horton letter confirmed that there was an order for 500 units, but only 457 units were delivered. The lowest serial number I've observed is TFA3675 and the highest TFA4115. All box ends observed have a Julian date span of 0115 to 0128 (04/25/1990 to 05/08/1990).
Your pistol fits within all these observations, however it includes a genuine round trigger guard and a different laser etching format (the serial number is below the model number, all other observed 4596 have the serial number above the model number).
If I was to guess, the original frame for your 4596 was somehow damaged and S&W replaced it under warranty with a later version of the 4506 frame. It would be interesting if Roy could find the repair record with your second request, if that is what happened.