To find the vehicle's consumption rate in miles per gallon, you can divide the total number of miles driven by the total gallons of gasoline used.
\[ \text{Consumption rate (miles per gallon)} = \frac{\text{Total miles}}{\text{Total gallons}} = \frac{90 \text{ miles}}{4 \text{ gallons}} = 22.5 \text{ miles per gallon} \]
Therefore, the vehicle's consumption rate is 22.5 miles per gallon.