Apparently if you break a block using a hammer, the fifth balloon shows up. If you do not have any hammers, I would email player support from within the game itself as you may get a quicker response.
finally got the 5th ballon on last move and had to use extra moves, which sux since is no mystery mountain,,,,wasted several hundred boosters before got the 5th...yes use the hammer for 5th but does not always work....basically a booster wasting level