According to Hustler's settings, most of mine were cut at 4.25" last week. I may have to move to 4.5" this week. That is where they look good. I don't check them with a ruler. I generally cut them at 3.75" until the lush growth starts, then move up from there. Some end up near 5" near the end of summer, then I slowly take some of the height out of them.
I do have a couple that are cut lower throughout the summer at the owners' request. But never below 3.5". Again, according to the mower adjustment.