4GHz is easy to get with the 1090T. I tend to run mine at 4.2GHz.
Highest I've reached is 4.6GHz which was barely stable enough to get a superpi run - though that was in my M4A79XTD EVO board.
With the 1055T 4GHz is definitely possible 24/7 as long as you have a board that can handle high HTT speeds.