Reasonable theory, however some significant differneces may be that which derails Murray's chances in this comparioson/projection:
Jim Courier did not take "forever" to win a major. He was 21years old and only 4 years into his pro career when he won the French Open in 1991.
Murray turned pro in 2005, yet he took the better part of a decade to win now--in 2012 at the age of 25.
Murray is older in tennis years, and theoretically has a shorter window of time remaining to win another three (if that). Unlike Courirer--who had a rather large body of legendary competition to deal with during his majors-winning years, Murray only had three legitimate threats.
Of course, competiton like Federer is fading in his twilight years (bolstered by Murray's Olympics beatdown of Federer)leaving Djokovic and Nadal as the only real threats, but again, Murray himself is older, so he may not win four.
It will be interesting to see how this plays out.