Cut mine about 3.5 and that keeps the weeds down well for me, mainly because I've got a lot of shade and they don't like that.
Dr. Jeff Krans was one of my turfgrass instructors at MSU way back when. He called St Augustine 60mph grass because "you've got to be driving 60mph when you drive by it in order for it to look good" . I like it though, only problem I ever have is some fungus/mold when we've been getting the rain we've had lately but even then it usually takes care of itself once it warms back up and quits raining.