If you're talking about the standard plastic ones then dakar is basically right. The 2wd has a small diameter inlet above the 'max' line for the turbo return pipe - about 8mm I.D., whereas the 4wd has a 16mm I.D. inlet below the 'min' mark.

Changing from one to another would need a step up or down in pipe diameter. Not sure what the significance of the different inlet heights is though - anyone else know?