Jolly G. Giant
I just bought a copy of DnD: WoA BG and its missing 5th page tile sheet from 13.
Fortunately, the 5th tile sheet is nearly identical to the 4th. The differences will be the actual dungeon tiles and 2 blade barrier tokens (identical to those on the 6th tile sheet).
Hopefully Wizards steps up and sends you a replacement. Otherwise worse case you could get by without it, especially if you purchase any of the other games. In that case the missing dungeon tiles won't be too bad as you could proxy from another set (sort of).