The fifth one is in the front of the frame just where the trigger guard merges in. It is sort of "on again, off again". It wasn't used in the first M&P, then was put in, then taken out again. When it was used, it allowed easier insertion of the cylinder stop spring.
Some folks count only the sideplate screws (so that is a "four screw"), some count the front screw as I did, some don't see the rear screw (which is covered by some stocks) and talk about two screws. I think if I have a stiff drink, I might be able to see a dozen or so screws.
Supposedly Smith put five screws in their guns - 4 in the side, one in front of the trigger. 5-screw. Then they discontinued using the top side screw, and put a little lip in there. 4-screw. Then they quit using the one in front of the trigger guard. 3-screw.