The LEGO City Arctic Outpost set 60035 is designed for builders aged 5-12 and has 374 pieces. It's a bit trickier than other sets for age 5+, such as the LEGO City Arctic Ice Crawler 60033 we reviewed a while ago, but still perfectly possible for our 4 1/2 year old.
The build is rewarding throughout, with instant gratification as you see the shape of your truck and trailer almost as soon as you start to build. It uses easy techniques and very few tiny elements, which makes it much less frustrating for a young LEGO fan.
Included is a quad bike which is a great little vehicle and my 4 year old's favourite bit of the set!