This one isn't a problem, its a game.

You need:

two players and 15 coins or buttons or bingo chips.

Put your coins in a line.  On your turn, you can remove 2, 3 OR 4 coins from the line.  Then, your partner does the same.

The goal is to take away coins and force your partner to take the last coin.

If you don't have 15 coins, you can draw 15 circles and cross them off instead of removing them.  You could play outside using chalk if you had it. 

Things to think about:

Does it matter who goes first?  Can you come up with a strategy for winning?