When might a Standard American bidder "bend" the five card major rule?
First, note that playing 4-card majors an opening of 1 Heart or 1 Spade is about 80% likely to be on a five card or longer suit. Thus players of a 4-card major system are already almost playing five-card majors. Bridge theorists and players for the past century have determined that even this slight difference in probability (80% vs 100% for a f+ card bid suit) is significant enough to require a significantly different tool kit for accurate bidding.

That said, there is then one simple pre-requisite for bending the always shows five or more cards rule:


You have no desire to ever play with this partner again, or with any opponent or kibitzer present.


The only exception to the above is when all of the above conditions are met:


You are in 3rd seat.
You do not have a no-trumpish hand.
You have thoroughly discussed with partner the circumstances and hand types which make this bid, and even more importantly those which do not make this bid, and properly summarized those on your convention card.
You are playing a Drury variant which allows responder to inquire on suit length and hand strength.