Thanks guys for confirming that they're all supposed to flash. I checked mine again tonight, the top left and right fireworks are working, the bottom left and right never flash.
I confirmed this with a few video searches on Youtube, too. When the other guys' fireworks are going off, all four flash.
Will have to see if that's a board issue. I checked the lamps and sockets, all seem good, haven't dug in with my meter yet.
I checked IPDB pictures regarding the coin box, it looks like it had one....? I asked the seller (Pinball Plus out of MN) and they said they'd keep an eye out but the boxes are getting hard to find.