Newer, and higher end cameras will have better high iso performance, so they will provide VASTLY better photos in low light situations. Higher end cameras will also have better autofocus systems, so they will be much better at getting shots in focus, especially of objects that are moving fast (sports) and in low light. On top of that, higher end bodies will have better metering abilities, so when shooting in a semi-auto mode they will likely put out more consistently accurate exposures.
The other thing with pro bodies, is that they are faster, have more external controls, and are built better/weather sealed. Now I know those aren't image quality characteristics, but they will give a photographer the ability to miss less shots due to fiddling with the camera, and they will allow you to go out and shoot in weather conditions that you can't with a lower end camera, which could be the difference between whether you get an image or not, period.
Now, if we're shooting at base iso, with the same lens, and all the same settings, taking a photo of a stationary subject in good light you would be very hard pressed to find a difference in the image quality of a D3100 and a D4 (assuming the image from the D3s was cropped 1.5x so that it matched that of the D3100). Pro bodies don't really produce better image quality in situations where the lighting conditions are perfect and the subjects aren't moving. What makes them better is that they can maintain great image quality in adverse conditions (low light, fast moving subjects, bad weather, etc) that would cripple the ability of an entry level body.
EDIT -- Just realized that we're talking about film bodies here, didn't catch that in the beginning. Disregard the high iso comments, but the rest still applies.