A solution that is 4 on pH scale is an acid. A pH scale is used to measure whether a solution is an acid or a base. the pH scale is numbered between 0 to 14. 7 is the neutral point of this scale. Any solution having a pH of 7 is a neutral solution. Acidity of a solution increases as the reading in the scale decreases from seven towards zero. The basic nature of the solution increases as the pH level of the solution increases from 7 towards 14. The acidic nature of a solution having pH value of 3 is ten times more acidic than a solution having pH value of 4. The same thing is same while comparing basic solutions as well.