Here's why:

* pH scale: The pH scale ranges from 0 to 14.

* Acidic: Solutions with a pH less than 7 are considered acidic.

* Neutral: A pH of 7 is neutral (like pure water).

* Basic (alkaline): Solutions with a pH greater than 7 are basic or alkaline.

Therefore, a pH of 4 indicates a solution that is significantly acidic.