I've always seen these called "I Spy" bags like the I-Spy series of books. I made one for my DD (also 4) for the holidays too. Actually, I started on it last year and never gathered enough fun stuff to go inside. I just finished it the other day. Here's a tutorial: http://rosie-baby-crafty.blogspot.com/2007/08/i-spy-bags.html
I finished mind by sewing a ribbon with a large jump ring on it. I attached the list of goodies to the jump ring. Here's an example of what I mean: http://www.etsy.com/view_listing.php?listing_id=8198102
If I get around to snapping a pic of DD's and figuring out how to add it here, I will.
Incidentally, I save plastic packaging from things like sheets, shower curtains, and the like and use those for the vinyl panel. Reduce, reuse, recycle....For the little things, cute buttons can go a long way. I bought packs of buttons at Joann's in themes (e.g. fruit basket with banana, orange, cherry, pineapple, grape, etc. buttons or Noah's Ark with a lion, chick, bunny, bird, turtle, etc.) I also used tiny dry food items (bean, coffee bean, small pasta) and office supplies/craft trinkets (paper clip, pen cap, bell, rhinesone, game pieces, puzzle pieces, etc).