I recently bought a used K20D, and when I tried my SMC A 28/2.8 on it (aperture ring set to A, of course

) I found that the camera wouldn't open the aperture up to more than f/4.0. It was no problem twisting the aperture ring all the way from f/22 to f/2.8, so the lens seemed fine. I then took the lens off, and found out that when I pushed the aperture actuator, it moved freely most of the way, but I had to use some force to push it that last bit to open the aperture up wide.
When I looked a bit closer, I discovered a piece of metal that the actuator stops against (see attachment), and when I push the actuator further, it pushes the metal piece out of the way, so to speak. Does anyone know if the lens is faulting, or if it's supposed to be that way? Is there any way I can fix it?