More often than you might expect, you will find yourself in fourth seat, after the first three players pass, wondering whether to open the bidding. In our research, we found that these decisions frequently won and lost lots of matchpoints. The guideline that we recommend is this (1) Open any hand that you would open as dealer; and (2) In addition, open any hand that satisfies the Rule of 15.
Here is the Rule of 15: add your HCP plus your number of spades; if the total is 15 or more, open this hand in fourth seat even though you would not have opened it as dealer.
However, some experts believe that, after the first three players pass, the only hands that should be opened are those that satisfy the Rule of 15 (even though you would have opened as dealer).