I am traveling solo to Sikkim for 14 days from 22nd December to 4th of Jan. I haven't made an itinerary yet because a lot of places (Gurudongmar Lake, Nathula Pass, Zero point and all roads going above Lachung) in North Sikkim could or could not be open due to heavy snowfall. If you're traveling solo, I suggest you stay in a backpacker hostel. You'd find a lot of fellow travelers who can suggest places depending on the weather then. Since you're just going for 4 days, you don't have to worry about missing out on places to see because you'd miss them anyway 😛 Sikkim is very beautiful in December, I'm sure you'd have a great trip!