The total physical output of a number of machines or workers is called physical productivity and is a function of the number of machines or workers. If P = f (x) is the productivity, dP/dx is the marginal physical productivity. If the marginal physical productivity for bricklayers is dP/dx = 90(x + 1)2, where P is the number of bricks laid per day and x is the number of bricklayers, find the physical productivity of 4 bricklayers.