Another thing to keep in mind with stopping down (closing down) the aperture. The rule of thumb for lenses is that a lens is sharpest 2 to 3 stops closed from its widest opening. For a lens with a maximum aperture of f1.4, it would be sharpest at f2.8 or f4. For a lens with a maximum aperture of f4, it would be sharpest at f8 or f11. Do you see the difference here? A lens that has a maximum aperture of f1.4 is going to let in 3 stops (8 times) more light than a lens with a maximum aperture of f4, at its sharpest setting.
Additionally, as was stated, as you continue to stop down to the smaller apertures, diffraction occurs, or the scattering of the light through the aperture, forming a wave pattern that reduces sharpness and image definition (it gets real technical but see
Diffraction - Wikipedia, the free encyclopedia ). So with an f1.4 lens, you have the best sharpness at say f4 and don't start getting into problems with diffraction until about f16, or about 5 stops with good sharpness and image definition. With the f4 lens you only get about 2 stops. Quite a big difference when you think of it.
Now you know why those fast lenses are so desirable!