I completed 4 hard level and had a reward option but then it disappeared. New option is that i have to start over again. there was nothing that said they was a time limit on using the reward.
Hi Se Boohar, this is a current glitch in the game. The game loses your progress when you log out. The only way you can complete this (until they fix the glitch) is to play all 4 hard Levels without closing the game!