6mth multi entry visa. Enter by date 25th Jan 2012. If leave and return 3rd Jan how long can I stay.

I am a NZ passport holder.My visa is a 6 month multi entry, category L. issued 25th July 2011. I arrived here in China 2nd October 2011 and will be travelling home on 22nd December. I need to return early January for approx 3 -4 weeks. I want to know if I need another visa or if the existing visa will cover me until end of January 2012.