The most important thing when building a slingshot is to find the perfect stick. That was the first thing we did. Although the perfect shape it started out a little big, but we can deal with that.
Try to ignore how dirty my child is and take a look at that perfect slingshot stick! We found it attached to a bush and lopped it off. After accomplishing step one we needed to cut the stick down to Gage size and add the additional parts. As we weren't up to trekking to the hardware store to complete our impromptu project we began to search house and shop to find the finishing parts.
We found one rubber band that was kind of small and some blue twine. So, we became creative. Cutting the rubber band and affixing it to the twine, which we than attached to the perfect slingshot stick, now the appropriate size. And bingo - we have made the perfect shooting machine! With just enough stretch from the small, single rubber band to make it effective, but limited in its shooting strength.
The perfect slingshot for a 4 year old. It shoots rocks about 3 or 4 feet and folded pieces of paper about 12 feet. A relatively safe toy that we made together with creativity and lots of fun. :)