The shift from the narrow vertical dispersion of the near field to the wider vertical dispersion of the far field with a four inch (.1 meter) high radiating plane will take place approximately .02 meters from that plane at 1400 Hz. You don't need to use multiple mid drivers to match that; a single 6 1/2" midbass would be quite adequate, and you only need be concerned if you're actually listening at a distance of .02 meters or less. As far as the narrowing vertical dispersion at progressively higher frequencies is concerned, that's a good thing; don't worry about it.