A 4-year-old might be a little young for this, but I've had pretty good luck keeping kids entertained by doing a quick plankton tow and showing them the results in a glass jar. I realize that not everyone has access to an "official" plankton net (it's one of the many advantages to being a card-carrying Marine Biologist, unlike this guy: clicky-clicky
), but you can pretty easily fashion one out of a wire hoop and the leg of an old pair of pantyhose (regular, fine-mesh ones, rather than the "fish-net" kind, SmackDaddy). It should look like this:
Just knot the end (so it doesn't still look like a stocking when you bring it back on deck) and tow it twenty or thirty feet behind the boat for five minutes or so. You may have to add a lead fishing weight or two to get it to sink properly. When you bring it back up, turn the net inside out as you dump the contents into a large glass jar full of sea water. The planktonic critters will last for an hour or more (at least some of them), as long as the water stays cool. The same thing also works in lakes, but there just isn't as much stuff to see (either in total quantity, or overall diversity). The results will look something like this:
Magnifying glasses will give everyone a better view of the critters (most are on the verge of microscopic), but tend to trigger Mal de Mar in some folks.