Nice that you are planning to visit Darjeeling in December. The sky would be clear and there are high chances of seeing beautiful views of snowcapped Kanchendzongha from almost every other place you go there..
The itinerary as suggested above,seems to be a perfect one. I would like to suggest a few small additions in that.
Day1/2: If you want to stay in Darjeeling town,you can check the hotels like Bellevue,Sunflower,Jagjeet/West Bengal Tourist Lodge (a big property) --which are bang on the Mall /Chowrasta (the main attraction of Darjeeling)..otherwise,if you don't have any constraint for budget,then I would suggest Mayfair/RJ Resorts/Windamere/Gymkhana etc)..don't forget to go on a leisure walk around the mall,via Rajbhavan,Mahaakal Temple--which is worth it!
Day3: On day 3,during your day trip to Mirik,you can visit a place called JORPOKHRI (consisting of two lakes on a hill top). Its an amazingly beautiful place which falls on the way to Mirik. Don't waste time at Pashupati Market in Nepal,which the driver may insist on...the road to Mirik,as said earlier,is one of the most beautiful hill roads of our country.
Day 4: In Kalimpong,you can also try to checj accomodation at DEOLO DGHC Guest House or Morgan House. Although,I guess there would be rush during this time of the year,but if you get to stay at DEOLO,it will be an experience of a lifetime,... While being there in Kalimpong,don't forget to visit the ARMY GOLF COURSE,its a beautiful place..