The number of moles of chloride ions in 250 ml of 4.00 m mgcl2

If you mean 4 moles per litre then:
In 1 litre there is 4 moles, therefore in .25 litres there is 1 mole.
1 mole of Magnesium Chloride would produce 2 moles of chloride ions (MgCl2 can also be written as Mg1Cl2, which dissolves to produce 1Mg(2+) and 2Cl(1-) ions: a 1:2 ratio)

So the answer is 2 moles of Chloride ions.        

Electrical charges of ionic bonds