The serial number for the TD-4KP is located on the underside (bottom panel) of the TD-4 module. It contains 2 letters and 5-6 numbers - (X6Y-12345). 
If you are submitting registration info, use the TD-4 module's serial number only. This represents the entire kit. 

Note: It is not necessary to list individual serial numbers for each pad or cymbal pad.