Beijing Airport to Tiananmen Square and Back in 4 hours?

Hey! My mate and I have 4 hours spare in between our flights (factoring in check-in etc.). Would it be possible to take the train to Dongzhimen and then the subway to Tiananmen Square, take a few photos, then head back in about 4 hours? If so, do you have any logistical advice in terms of how to transfer between trains and subways etc.? Thanks!