Difference between infant and toddler shoes?!?

My son is a year old and now that Fall is here he cannot be shoeless lol. He wears a size four shoe...but whats the difference between toddler and infant sizes??? I'm so confused. would I be looking for infant shoes or toddler shoes?? help! thanks so much!
Update: the smallest "toddler shoe" I can find in DC's are fives
Update 2: Well with socks he may be in size fives...I didnt know the difference lol thanks. LJ isnt walking yet but he is nearly there and I know he will be this winter
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