I feel the M4 design was made to be more versatile and reduce falses but keep sensitivity towards real threats. The M4 may be cheaper to make but that doesn’t necessarily mean it’s a lesser horn than the S7.
I think the M4 on the 9500I uses a diffrent type of receaver and there are many to choos from depending on the aplication. A better horn that is more versital and can be used for many diffrent aplications will cost less because more are purchased.
See my description of the 9500I’s sensitivity & filtering system:
Description of the Escort Passport 9500I sensitivity & filtering system