First, the Molarity equation is M = mol/L. However, the problem is looking for the TOTAL NUMBER OF MOLES in which will have you to reposition the said equation: mol = M/L. Observe that you need L not mL, so you need to convert 250 mL to L that will results to .25 L. Then, substitute the value of the units to the formula and you should be able to get 4 moles.