CHEM Mole Concept！！

1 mole of atom has 6.02 x 10^23
1 mole of Re has 6.02 x 10^23 atoms and you are given
1 mole of Re has 186.21g

That means:

6.02 x 10^23 atoms weigh 186.21g and using ratio calculation, the mass of 1 Re atom is: 186.21/(6.02 x 10^23) = 3.09 x 10^-22g. Multiply by 4 to give the mass of 4 atoms 1.24 x 10^-21g.

Hope that is useful :D