When taking my social security benefits at age 62, I did not receive credit for my 4 years of active duty military service from 1960 to 1964. Can I retroactively pick up these benefits now?
Possibly. Your benefit rate could certainly be recalculated prospectively in order to consider any military credits for which you weren't given credit, but whether or not any resulting increase could be paid retroactively may depend on the cause of the omission. Administrative finality bars correction of Social Security determinations after the passage of 4 years unless an exception applies, such as a clerical error (https://secure.ssa.gov/poms.nsf/lnx/0204020080).
What you'll likely want to do is submit proof of your military service, ideally form DD-214, to Social Security along with a written request for a reopening and recalculation of your benefit rate.