Yes, the little ones will be able to see the fireworks fine from the 4th floor, and if the speakers are working the music will be piped in. You will still get plenty of noise from the fireworks, but not nearly as much as being in the park.
If you get done with dinner a little early, but still don't want to fight the Main Street crowds, another option would be to ride the monorail around to the Poly and watch from the Poly beach. Or, ride the monorail over to the Poly AFTER the fireworks and work your way to the beach to watch the Electrical Water Pageant.