How to pronounce "论" I"ve been trying to figure out the tones for the character 论. It seems like it is fourth tone in 讨论. However, in something like 一概而论 I'm not entirely sure. Is this a taiwan/Mainland division? Or is it decided on a word by word basis?
It's the 4th in 一概而论, both regions: 

Check. e.g.: 


Mainland: 在线新华字典 
Taiwan: 教育部重编国语字典


(Both URLs point to the '一概而论' entry)

论 does have a 2nd tone reading, but 论语 seems to be the only where it is used.