Pottery Plant Co. has entered into an agreement to buy its actual requirements of potting soil for six months from Green Supplies, a garden supply company. Green Supplies has agreed to sell all the potting soil they will require for six months. The agreement between the two companies is:

1. Unenforceable because it is too indefinite as to quantity.
2. Unenforceable because it lacks mutuality of obligation.
3. Unenforceable because it is illusory.
4. A valid and enforceable contract.