is there a way to control wether using 2x or 4x?
I don't think so... it's either ON or OFF, and probably uses your default value that you have in your drivers. To be sure you have do tests like rendering & saving a screen produced by Blitz3D and then forcing all AA options in turn from the drivers, and then compare those rendered Blitz3D screens to the one you got when you used Blitz3D AA=ON.