Tom divided 15 coins, each 1p, into four small bags.

He labelled each bag with the number of 1p coins inside it.

After doing this, he could then pay any amount of money from 1p to 15p by just using the certain bags without the need to open any bags.

How many 1p coins did Tom put in each bag?

Is there only one way Tom could have done this? If so, why?