"Five card majors" is the foundation of the Standard American system. Yet rules are made to be broken under special circumstances.
My understanding is that some bidders will adhere to "five card majors" in first and second position, but bid good four card majors (say, KQxx or better) in third and fourth position. The rationale appears to be that if your partner has already passed, you are the "captain" of the team, and the cost of misleading partner is less than if partner hadn't spoken. In this case, partner may use the Drury convention to find out if you are bidding a four or five card major.
Another variation I've heard is that "five card majors" must apply to hearts, but need not apply to spades (with or without the third and fourth seat variation referred to above). The reason is that a bid of x spades beats out a bid of x hearts, but you need to go to x+1 hearts to beat out an x spades bid. So there is a greater need of the fifth card in hearts.
Which experts or variations basically follow Standard American but allow deviations from "five card majors" under particular circumstances?