Each driver has a crossover to only handle its frequency range. Since the tweeter is only handling the treble range, adding a midrange driver only handling the midrange frequencies will not impact the overall average impedance of 4 ohms across the spectrum.
What I want is vocal clarity as I'm not looking for a hi-fi installation what with my noisy engine and the poor cabin environment of any car. The Focals are good but the door units are not bass monsters and the top-dash tweeters are considerably better than the stock item but leave a hole in the midrange.
After all, when I get home I have the big rig to give me what I want...