Sounds to me like he already has one. ;D
There are worse things in life than guys who love feet.
I agree with rose it sound like he's going through a phase and possibly has a foot fetish. I wouldn't think too much of it as kids that age are constantly learning and evolving.
Feet are weird things. They're like hands, but not. They have strange little wiggly worm toes. How can he not be curious about them? I mean, take some time and play with his feet. When you're learning about the world even the littlest things us adults don't think twice about are such mysteries.
I doubt it's a foot fetish, just something new for him to explore.
...its deftly a "thing" with him... I hope he grows out of it. I explained to him that what he is feeling is normal but needs to be done in the privacy of his room or shower...can't believe I had to explain that to a 4 year Old.