Adding an additional set of speakers of the same type to those already in place makes for bad sound.  The graphs illustrate what happens when 4 speakers of the same type are covering the same area.  Using two speakers gives even coverage for the entire audience (graph right).  By making the mistake of adding a second pair, you may create multiple 20db nulls (cancellation of sound) throughout the critical vocal frequencies for a large part of the listening area (graph left), while only experiencing an overall gain of 3dB in volume.  That’s not a very good tradeoff!  Since EQ cannot correct this (and the attempt will fry some members of the audiences’ ears), the correct approach is simply to increase the volume of your mains by 3dB if you have it, or be content with providing the best possible sound for as much of the audience as possible.  And I understand that it doesn’t look as cool.