how many molecules of h2o are there in 4.0 mol of h2o?
If you have 6.022⋅1023 molecules of water, then you can say for a fact that you have 1 mole of water. Now, you know that your sample contains 4.0 moles of water. In order to find the number of molecules of water it contains, you can use Avogadro's constant as a conversion factor.