pH is a logarithmic scale used to measure the acidity or basicity of a solution. The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration ([H+]) in the solution, and it ranges from 0 to 14.

A lower pH value indicates a higher hydrogen ion concentration, meaning the solution is more acidic. A higher pH value indicates a lower hydrogen ion concentration, meaning the solution is more basic.

The difference between two pH values can be used to compare the relative acidity or basicity of two solutions. The larger the difference in pH, the greater the difference in acidity or basicity.

In the case of pH 9 and pH 4, the difference between the two pH values is 5 (9 - 4 = 5). This means that pH 9 is 10^5 (100,000) times more basic than pH 4.

Therefore, pH 9 is much stronger than pH 4.