6" is is very nice... but 4" of +/-50 k/m3 rockwool will be fine [and much more efficent per unit cost].
Check out the Roxul Rigid categories for 4" / 100mm panels - don't get drawn into the density = performance myth.
http://www.bobgolds.com/AbsorptionCoefficients.htm
Product thickness density 125hz 250hz 500hz 1000hz 2000hz 4000hz
RHT 40 4" (100mm) 3.5 pcf (56 kg/m3) 1.07 1.01 1.07 1.06 1.07 1.16
http://www.roxul.com/sw16796.asp