Hi:
Is this something you'd want to use in a rule? If so, you set conditions for your rule. Say, for example, you have a field called field2 that you want to set with the value from your if statement. You'd first add a condition for field1 > 12. Your rule would have an action to set field2's value to field1 div 4. Then you'd add another rule, with another condition -- this one would be field1<= 12. The action for this rule would be to set field2's value to 0.
If what you want is a conditional default value, there is nothing out of the box for that -- there is some XPath magic you can use to create a conditional default, but it can be a little tricky.