4. Change the word sentence into a math equation. You don't have to solve the equation. $21 \%$ of 43 is what number? Write your answer in the space provided.

Step 1 ：The question is asking to convert a word sentence into a mathematical equation. The sentence is '21% of 43 is what number?'. In mathematical terms, 'of' usually means multiplication, and 'is' usually means equals. So, we can convert this sentence into the equation '21% * 43 = x', where x is the unknown number we are trying to find.

Step 2 ：The mathematical equation that represents the sentence '21% of 43 is what number?' is \(\boxed{0.21 * 43 = x}\).