Form To Open New Window With Alternate URLs?

I am trying to create a form which when the submit button is selected will open a new window with 1 of 4 URLs based on the the combination of data in 2 select boxes.

For example:

if select box 1 = a and select box 2 = a then open new window with URL 1
if select box 1 = a and select box 2 = b then open new window with URL 2
if select box 1 = b and select box 2 = a then open new window with URL 3
if select box 1 = b and select box 2 = b then open new window with URL 4