MonQuest reminds me of an RPG about Xialoin martial artists, warrior chimpanzees, or both.
And Mongoose Quest would be weird; "Oh, where of where could my mongoose have gone? Oh, here it is!"
MRQ or RQ4 is fine for me. But does pose an interesting question of what to call the variant games that are going to inevitably result from Mongoose's Open Content declaration. Including "Quest" in every title would be annoying -- especially since there are other games out there with no relationship to RQ that use that naming convention.