I've read (Anandtech, among other places) it was actually the Qualcomm chip's fabrication size that was the reason the iPhone4S didn't have it. It's too large to fit comfortably into even the current size of the iPhone 4/S; and they want to go slimmer in the iPhone 5 apparently (which was Steve Jobs last labor of love at Apple).
Tim Cook stated that current LTE modems would force Apple to make design decisions Apple didn't want to make. That pretty much sealed the 4S not having LTE.