Here's why:

* The pH scale is logarithmic. This means that each whole number change in pH represents a tenfold change in acidity or alkalinity.

* Lower pH values indicate higher acidity. A pH of 0 is the most acidic, while a pH of 14 is the most alkaline (basic).

* A pH of 7 is neutral.

Therefore, pH 4 is much more acidic than pH 9.