To find out how much the plumber charges per hour of labor, you can divide the total amount paid by the number of hours worked.
Dylan paid the plumber $120 for 4 hours of labor:
\[ \text{Charge per hour} = \frac{\text{Total Amount Paid}}{\text{Number of Hours}} = \frac{120}{4} = 30 \]
So, the plumber charges $30 per hour.