Since you are arrving at Beijing at mid night, I think you have to take a taxi to your hotel from the airport, and the same as that of your leaving. You can make your itinerary as follows:
Apr. 1, take a taxi from your hotel to Tiananmen square to visit the square, forbidden city and temple of heaven.
Apr. 2, take the tourism bus from Qianmen to Badaling Great wall, and then take bus no. 919 from Badaling to Changping Xiguan to take bus no. 314 to visit Ming Tombs.
Apr. 3, take Beijing subway line 2 from Qianmen to Chongwenmen, transfer to subway line 5 to Huxin West street south entrance, again transfer to Subway line 10 to Beitucheng, change to subway line 8 finally to Olympic sites.
Apr. 4, take bus no. 690 from Qianmen to visit summer palace.