We plan to travel with kids to Chengdu late Dec this year, what do you suggest we do in 4 days?

Well, you can arrange the sightseeing in the following way.
Day 1: Chengdu Panda Breeding and Research Center, Wuhou Temple and Jinli
Day 2: Happy Valley, Wide and Narrow Alley
Day 3: A side trip to Dujiangyan or Mt. Qingcheng
Day 4: Jinsha Site Museum, Chunxi Road

The above popular scenic areas will highly attract kids' attention. :-)

Considering you are going in Dec. I’d suggest you add the snow sightseeing and skiing into your itinerary. If you are interested, you can arrange your kids to go to the Xiling Snow Mountain and ski there on Day 3. For the best experience, it is best to visit during mid or end of Dec.

We live near Chengdu. I would only recommend Jinsha if your kids are older and also interested in that kind of thing (excavations, history.) My husband and I enjoyed it but younger kids would probably find it boring. You could do a day trip to Leshan/Emei. The Global Center (with waterpark) is a warmer alternative to Happy Valley. Make sure you take time to sample food. The food and Pandas are what the area is most famous for :).