What can you do about stinky feet four year old?

Don't . Wear them 2 days in a row and many antiperspirant are available. Caused by too much moisture so important to wash and dry feet well.
Check the shoes. Many times the kids shoes become a breeding ground for odor producing organisms. If they are too stinky, and tolerant to a good cleaning, a good wash or a soak in listerine may cancel out the smell for a good while. A good foot scrub can help temporarily, but the footware are important.
'Stinky feet' Usually this is caused by a form of athlete's foot, which can be treated with over the counter sprays, like 'odor eaters' sprayed on the feet, especially between the toes, and into the frequently worn shoes.
Work at it... First off, make sure your feet stay dry and clean. Wash between the toes each day and wear fresh socks. •acrylic cushion sock fabric maintains resiliency and provides a more natural yielding, forgiving, dynamic environment for the foot (like walking barefoot in grass or sand). •result: more protection, more comfort. Secondly, use drying agents directly on the foot, like certain-dry or drysol.
4 yr old stinky feet. I think the thing to glean from the previous answers is that there are several approaches to correcting this. Fungus is a factor, as is hygiene and shoes are a great place for bacteria and fungus to hide out. There are sprays for shoes - but let it dry and don't wear the shoes for at least a couple days as little one's skin is more sensitive than adults. See your podiatrist for further help.