How do you find the name of the chemical formula pb3n4 (the 3 and 4 are subscripts)

This is a salt because there is a cation and an anion.  Because lead can have multiple charges, its charge must correspond to the charge of the anion.  The charge on the N is -3, so the 4 N's equal a total charge of -12. Therefore, the total charge of the 3 Pb's must be 12, so each lead should have a charge of +4.  So the name of the compound is Lead (IV) Nitride. 

Copper, silver, aluminum, cobalt, chromium, mercury