First, congratulations!
XDY is the manufacturer for O'Day, so no info to glean from that. I don't know what body vs means, but Google seems to show it associated with other random types of boats, so I'm guessing that's not indicative of shoal or deep draft.
I believe O'Day only made the 28 with a centerboard into the early 80's. We bought ours in 85 and at the time the two options were the shoal draft and full keel. We had the shoal draft at 3'8". I agree with Tally Ho that the seller probably rounded 3'8" to 4'. Although it is possible that they preferred to round down to make the boat more appealing in a shallow area.
O'Day probably did keep records of which hulls were which configuration, but I don't know if anyone would still have access to those. About the only resource I can think of for such things is Rudy at D&R Marine,
http://www.drmarine.com/. Rudy was the production manager, or something like that, for O'Day, and generally if you can get him on the phone the reports are he's pretty helpful.