質: when to pronounce in 2nd tone or 4th (Mandarin)?
You should always pronounce the 4th tone in the standard mandarin. 

Sometimes, I heard people wrongly pronounced the 2nd tone in the words of 质量，质地, which is probably acceptable to a native ear. But again that's wrong pronunciation.   

But if you pronounce the 2nd tone for 质疑，品质, it's really bad (at least to me). 

An interesting observation, in a online dictionary, I search for 质地， and it's marked the 4th tone for simplified and 2th tone for traditional one.  So, I doubt this could be regional thing as fefe suggested in his/her answer.