To determine the price of one box of cereal, we first need to find out how much money Andrea spent on the cereal.
Initially, Andrea had $43.50. After buying the cereal, she had $27.70 left. We can calculate the amount spent on the cereal as follows:
\[ \text{Amount spent on cereal} = \text{Initial amount} - \text{Amount left} \] \[ \text{Amount spent on cereal} = 43.50 - 27.70 = 15.80 \]
Next, we know that Andrea bought 4 boxes of cereal for a total of $15.80. To find the price of one box of cereal, we divide the total amount spent by the number of boxes:
\[ \text{Price of one box of cereal} = \frac{\text{Amount spent on cereal}}{\text{Number of boxes}} = \frac{15.80}{4} \]
Calculating this gives:
\[ \text{Price of one box of cereal} = 3.95 \]
Therefore, the price of one box of cereal is
\[ \boxed{3.95} \]