title: Validate zip code using just first 4 digits
body: <p>I'm using table rates to assign shipping costs in Magento.
  Our zip codes are in the following format: XXXX-XXX. Is there any "easy" way I can change the validation so that 1234-234 is assigned the same shipping cost as 1234-678 without doing it explicitly? 
  I would like to fill the tablerates.csv file like this:</p>
  
  <pre><code>Country,Region/State,"Zip/Postal Code","Order Subtotal (and above)","Shipping Price"
  US,*,1234,20,5
  </code></pre>
  
  <p>and the system would assign 5 as the shipping cost to 1234-234, since 1234 is contained within 1234-234.</p>
  
  <p>I'm imagining a simple change from "if clientzip = csvzip" to "if clientzip like 'csvzip%" somewhere in Magento's files...</p>