On arrival VISA issue to USA Nationals

Self & Wife are USA Nationals on vacation in India. We are transiting through Hong Kong on our way back to USA on 17 Aug 2013. We are taking a break at Hong Kong to travel to Beijing on 17 Aug 2013. We will stay and take tours at Beijing for 3 days(18, 19,20 Aug) and go from Beijing to Siem Reap on 21 Aug 2013. We will stay at Siem reap for 3 days (22,23,24 aug) and return to Hong Kong on 25 Aug 2013. On Aug 26 we will leave to USA.
(a) Can we get VOA at Hong Kong for break period
(b) Can we get on arrival tourist one at Beijing for 4 days.