4 Story Tower Cubes

I decided to give it to my girlfriend to see how many combinations she could come up with the cubes. She first began with making a tower of orange, then started replacing one orange with one black. After she was done with all the combinations by replacing one cube, she began replacing two orange cubes and then 3 orange cubes. The last piece she formed contained all black cubes. At the end she formed 15 different combinations. I asked her how she knew she was done and she responded that she did not know. My question made her second guess herself and she looked at all the combinations she had made. Within a few minutes, she figured out that she missed one.