The only reason there is a different model number for each iPad and iPhone 4 is because of the different cellular technology in them. AT&T uses GSM, Verizon uses CDMA, and then there's the WiFi iPad, same thing with the two types of iPhone 4. The iPhone 4S however is a world phone and can work on every carrier, so it's only one model.
EDIT: I don't think the Sprint iPhone 4 would have a different model number.
In the story posted this morning about the iPhone5,1 in the data string it shows an iPhone4,1 and iPhone4,2. Why are there only 2 iPhone versions on three diff. US carriers but now a supposed iPad 5 shows up?