It is possible that it was the last four numbers of his US or RA serial number, as opposed to his SSAN.
I entered AD in June of '69 and was issued a RA #****4823.
It was used exclusively for a few months, until such time as the SSAN replaced the RA/US/NG/AUS numbers. Prior to that, the SSAN, while a part of your records, was not used as an identifying number for DOD purposes. I do not think it was put on dog tags until about mid-'70's
So, remember that when searching data bases...the number may not match his SSAN, but could match his Service Number (SN).
I still have a pair of OG trousers with the first letter of my last name and the last four numbers of my RA number, stamped onto the small flap that covers the seam at the back of the trousers.
In fact, they're stiff as a board from the starching they received, which common in those days.
Probably dates back to about mid-1970.
Good luck with the search.
It was used exclusively for a few months, until such time as the SSAN replaced the RA/US/NG/AUS numbers.
I went RA on June 30, 1969 and was given an RA number.
Next day, July 1, 1969, DOD went to the SSN as you said.
Funny thing, as much as they drilled into us, "always remember your service number!", I still remember my RA number 40+ years later :-)