With the pressure at 4, you can actually breathe perfectly normally, but you inhale more of your own CO2. The CO2 is part of the feedback mechanism that tells your brain that you're not getting enough air.
Your mask leaks air out of the exhaust vent. This is what keeps you from rebreathing the same air over and over. At 4 cmH2O, the vent flushes out enough CO2 and air to be safe, but it may feel uncomfortable. You could design a CPAP mask to leak more air at 4 cmH2O, but then it would leak out a lot more air at higher pressures.
Try this experiment if you have a nasal mask, and are feeling suffocated. Inhale through the mask, but exhale through the mouth. See if the suffocating feeling goes away.
It would be interesting if manufacturers made masks that either had some sort of "smart" vent that leaks more at low pressures or had an adjustable vent that could be set for use at low pressures.