Hi we are in Uetikon as well, you will struggle to find a swimming class for a 4yo here, they normally want minimum age to be 5yo. Having said that your best bet is in Meilen, our son is just finishing the 1st level new classes start in August after the holidays I think. The advantage of Meilen is the ratio of instructors to students about 1:5, a woman told me her daughter`s class was one instructor for ten students which does not give much 1:1 time in a 25min lesson. Zollikon also has swim classes Sat mornings and I think they will accept 4yolds.
http://www.scmeilen.ch/
Another link to useful local info (in German)
http://www.goldkueste.ch/badi_meilen.html