I just finished level A with a 4yr old. He was interested so I thought I'd give it a shot. He loved it! He'd jump up and down when it was time to do math! We played games as needed and his retention has been great. He's now in level B. I didn't skip as many lessons as recommended when you move from A to B since he is so young, I figured more review wouldn't hurt and he hasn't minded the review.

I say go for it....watch for signs of frustration and just camp out and play games until he is ready to move on.