Those in four-star positions usually rotate out every four years. Nominees for these positions (as well as three-star positions) must be approved by Congress. They are nominated for two-years and usually are re-nominated for another two years (hence the four-year tour).
Forgot to mention: he assumed his position as Director, NSA / Commander, US Cyber Command / Chief, Central Security Service in April 2014. So this is the normal four-year rotation of commanders mentioned above.