I must be missing some too. I got the other three location trophies. Since then I've visited teramorph and unlocked those locations and also the optional areas at caustic caverns (minecart, minecraft, & lost treasure side quest locations). Must be missing some others.
I found the last places I was missing. Fast travel to southern shelf and go to the area where midget was on the Skagg you fought before sanctuary. There was two undiscovered locations with Rakks and Bullymongs way up away from anywhere a quest would take you.