Is it enough if I take a taxi from the train station to Beijing International airport within 4 hours?

Hi,
My train from Shanghai arrives in Beijing on April 19 at 7:30 am. My flight from Beijing International airport leaves at 12 pm on the same day. Will I have enough time if I catch a taxi from the train station at that early hour to the airport or will the traffic be too bad? Also is it easy to find a taxi at the South Station or will it take a long time. Thank you.