That sounds correct. I think the acid of carbon dioxide has a pH constant of about 3.88, so that should be the absolute minimum pH it can reach with CO2 alone.You probably mean to say that RO water has zero KH, and therefore its pH may swing easily.
Usually it grabs carbon dioxide from the air and becomes slightly acidic. Probably down to pH5.5.
But I think there is no way it will go down to pH3-4, that is too acidic: pH value is based on logarithmic scale depending on the H+ ions concentration.