We have tried lots of different types of sprinklers this summer for our 4yo, but they were all big disappointments. We actually purchased the Little Tykes from a local stuff-mart, and it is fab!
There are no moving parts, which is great because there is nothing to break. The water shoots out at just the right height so that my small 4yo can play without getting water shot straight in the face. (He *hates* that!) The balls are really fun when you get the water pressure just right, and you get some serious hang time!