Customer bought more than the stock has, how?

Our stock has only 4 items and the customer could place an order for 9 items (the stock showed as -5 now), how could this happen and how can I prevent it from happening in the future?
Thank you

What are your settings in the Admin> Configuration> Stock ?

Check stock level true
Subtract stock true
Allow Checkout true
Mark product out of stock ***
Stock Re-order level 5
Products status in Catalog when out of stock should be set to 0
Show Sold Out Image in place of Add to Cart 1
Product Quantity Decimals 0
Show Shopping Cart - Delete Checkboxes or Delete Button 3
Show Shopping Cart - Update Cart Button Location 3
Show New Products on empty Shopping Cart Page 1
Show Featured Products on empty Shopping Cart Page 2
Show Special Products on empty Shopping Cart Page 3
Show Upcoming Products on empty Shopping Cart Page 4

Above is my current config. Should I set allow checkout to false?

Should I set allow checkout to false?

Yep, that will tell the customer there is not enough in stock and that they will have to adjust the quantity in their cart.