Why is 4 weeks anything special? Why not 3? Why not 5? Anything special about 4? Is there data supporting 4 being the right number?
im sure that at the time (1970s), in those market conditions, 4wks tested better and or performed better than the others. There is nothing special about the 4wk mark...the only number that should matter is the one that tests best for you, then try in real time to refine further.