I did some research online and also called my dealers parts department... at great personal expense I might add. (It's long distance. 10 miles away. Can you believe that?)
OK, according to my parts people they will probably not interchange w/o "at least some" modification, if they would at all. I made sure he knew what I was asking and he looked up the part numbers for the left side of a 4dr. S and a 5dr and came up with two distinctly different part numbers. Then he looked up the R.S. for both vehicles and came up with the same thing.
In talking with him, he stated that the two have so many differences that they are almost two completely different cars!! I was a bit surprised by this statement, though I knew he was talking mostly about the bodywork.
Before calling them, I went on the Mazda N.A. site and looked up Accessories. Under "exterior" they have the lower door molding set, the mate to the side sill extensions, and they too were different part numbers for the 4 and 5dr models.
I then checked out the MazdaSpeed accessories as they have a deeper side sill "add-on" extension. It was ONLY available for the 5dr.
'Sorry Buddy, but I think you're OOL with an interchange on this one. (protest)
'Sorry for being the bringer of bad news. I know it's not what you wanted to hear.