In comparing the two face cushions, I noticed that the firmer inner layer of silicone on the Mirage Quattro comes together much closer at the nose bridge than on the Quattro FX. That is by design as the Quattro FX according to Resmed's fittings guides should be placed on your nose mid-bridge where your nose is wider, then swung down over your mouth, their so-called "swing and seal" technique in order to get a good tight seal. The Mirage Quattro fits higher up on the nose bridge where you nose is narrower. My belief was that the narrowness of the firmer inner layer was what was causing my nose bridge sores.
So, I took some scissors to the face cushion on the Mirage Quattro and very carefully trimmed the firmer inner layer of silicone on each side in order to widen the amount of space at the nose bridge, similar to that on the Quattro FX. Lo and behold, no more nose bridge sores!!!
So, if you're experiencing nose bridge sores with this mask or others, you might give this mod a try. One word of caution though is you need to be very careful when trimming the firmer inner silicone layer because the thinner outer layer of silicone is very easily punctured by the tip of a scissors. I know because the first time I tried doing this, that's what happened and once the thinner layer is punctured or cut, you'll never be able to get a seal at all.