I'd say it takes about 5-6 minutes from the CR lobby exit (that's if your dh really likes to move, as mine does). I've heard others say it takes longer, and your 4 y/o might not be able to keep up with that kind of pace. But it's such a great walk! There are the topiaries to check out, the personalized bricks on the Walk around the World, and the MK attractions getting closer and closer as you walk. Once you leave the lobby, MK seems VERY close (almost as if you're there already).
The monorail is probably faster IF you don't have to wait for it. If one has just left, walking is definitely faster. And if the monorail is crowded, I'd say walking is much more pleasant. The two things we loved best about CR: the view of the MK from the Tower (absolutely can't be beat) and being able to walk over.