* Know the conversion: 1 liter (L) = 1000 milliliters (mL)

* Set up a proportion: (4.5 L) / (x mL) = (1 L) / (1000 mL)

* Cross-multiply: 4.5 L * 1000 mL = 1 L * x mL

* Solve for x: 4500 mL = x mL

Therefore, there are 4500 milliliters in four and a half liters.