We need to write a balanced equation for the formation of ammonia from nitrogen:
Nitrogen and ammonia are in a 1 : 2 ratio i.e. for every mole of nitrogen used, two moles of ammonia are produced.
Since we used 4 moles of nitrogen in this reaction, 8 moles of ammonia are produced (4 x 2).
Now we just need to convert the number of moles of ammonia into the mass of ammonia.
We do this using the following equation:
Mass (g) = Mr (g/mol) x Number of Moles (mol)
Mr ammonia = 17.031 g/mol
Mass = 17.031 x 8
Mass = 136.25 g