I saw it with my 4 (nearly 5yo) DS this afternoon. He enjoyed it but then he is quite brave compared to some of his friends. The main characters are all quite 'dark' really in terms of their animation, he isn't a jolly, smiley Santa for instance and their are some horses (the 'nightmares') that wouldn't be out of place in Lord of the Rings. Nothing he was watching worried me and all the other children there were around his age.
Just watched this with my kids (10, 8). I loved it. The animation is spectacular (the sand!), and the message is believe in yourself and others will believe in you, which is a good one. For me it's second only to UP as far as Disney/pixar/DreamWorks films go.
Saw it in 3D yesterday. 9-15 year olds all loved it; sister and I loved it; dp hated it. Not sure for a four year old, but agree that the animation is beautiful and in 3D it was absolutely magical to look at.