A monopolist producing only one product has two plants with the following marginal cost functions: MC1 = 20 + 2Q1 and MC2= 10 + 5Q2, where MC1 and MC2 are the marginal costs in plants 1 and 2, and Q1 and Q2 are the levels of output in each plant, respectively. If the firm is maximizing profits and is producing Q2 = 4, what is Q1?