I put some travel times together from the NC ferry schedules and Google. The Cedar Island ferry is a little less than 50 miles from AB, but it is mostly 2 lane state roads. Allow yourself enough time to get there.
Travel times:
ferry - Cedar Island to Ocracoke Island: 2H 15M
ferry - Ocracoke Island to Hatteras: 0H 55M
Auto - Hatteras to OBBC, Kill Devil Hills: 1H 30M (about 67 miles)
Unknown:
Waiting time for Ocracoke Island to Hatteras ferry; ferries run every 15 -20 minutes. Saturday is a high usage day, expect a long wait.
Travel time from Hatteras to OBBC, Kill Devil Hills is given here without Saturday traffic. For about ¾ of the distance you will be traveling on Rte 12 which is a 2 lane state road which has a temporary bridge just north of Rodanthe.
Adding up the known travel times, you won't be getting there (OBBC) by 4PM, including time for the unknown, waiting time and Rte 12 travel time, my SWAG would be 6PM. And this does not include stopping for lunch anywhere.
SBtS