At LEAST every two years, climate dependent.
Here in Colorado, it's dry, if you're in humid areas, either every year, or don't run it at all, unless you're specifically tracking your car, and then you're going to be refreshing every session anyway.
DOT4 is much more hydrophilic than DOT3.
I run it because I occasionally track it, but more so because I do so much brake bedding, to the point of fade.