My m750 shock is getting weak but i have a mint spare S4 shock will it fit the m750????
It will bolt on, but the linkage ratios are different. It won't damp the way you want, and the spring won't be right.
Not sure what year your bike is, so this may be completely irrelevant, but if you have a pre-2002 bike and the S4 shock is the same as the 2002 Monster shock then it may not even bolt up. It will look just right until you try to put in the top, then you'll discover that the top bearing is around 2mm thicker than the original, and it won't even fit in. Ask me how I know... Last time I buy a part off eBay without searching the forum first!
Nevertheless, ducpainter's point about the incorrect spring/damping characteristics due to different suspension geometries is really all you need to know.
And if you have a 2002+ 750, then I might just have a gently used shock from an 02 750ie sitting in my closet to sell you...
If he's got an '02 M750, it'll bolt right in.
Otherwise, no.