Hmmm, 4 yr old and fishing, well it is my experience that ought to take about 30-40 seconds till they get bored and want to move on.
If you want to combine fireworks and fishing, you take a hand grenade, wrap a layer of toilet paper around the safety lever, pull pin and drop in deep water, then get the (insert colorful Word)
out of there. Resulting boom will cause easily retriveable fish. Ah heck maybe you better not try that one. Someone will be upset. A simple worn on a hook is probably better. Any way good luck