I think that 4 (ok, almost 4) is the perfect age to enjoy fireworks! She loved them. And every single one of them was "wonderful. Oh, wonderful!" We brought Grandma with us, and that made it even more fun.
Last year, she was terrified. she'd watch them, but she was in my lap and holding on for dear life. this time, there were oohs and ahhs, and wonderfuls, and such.
I had to work from 6:30-7:30, then we grabbed a bite to eat at the dear old Wendy's (their toys are always soooooooooo stupid! What's a 3 year old going to do with a computer disk? Especially while she's watching fireworks! And their 3-and-under toys are always stupid little frogs (which we already have... but I digress). We picked the perfect time to go to Wendy's, because it was pouring rain! We were worried it'd ruin the festivities. I think it actually helped (not as many people as there would have been otherwise, great for the firefighters, and pyrotechnitians), and it didn't rain on us while we were there. It meant the ground was a little wet and the air was a little brisk, but welcome to Idaho! Especially this year.