It isn't, but 4 is.

2 actually is possible. I have even achieved it and that was without excessive training. At least, as I discovered later, without excessive training while I was awake. On a related note it is also possible to sustain 1 breath per 50 meters while running.

With both of the above it isn't a matter of mere psychology. The changes may have to occur down to the physiological level, along similar lines to altitude training. The oxygen absorption of the blood itself is altered.