The formula of the complex anion hexacyanoferrate(II) is [Fe(CN)6]−4. The oxidation number of iron is +2 and each cyanide ion has −1 charge. Hence, the net charge on the complex anion is +2+6(−1)=−4.
-4 charge on complex anion will be balanced by +3 charge on Fe3+ cation when 4 Fe3+ cations combine with 3 [Fe(CN)6]−4 complex anions.
The formula of the coordination compound Iron(III) hexacyanoferrate(II) is Fe4([Fe(CN)6])3
Note: In the name Iron(III) hexacyanoferrate(II), the numbers in parenthesis in roman numerals indicates the oxidation number of iron atoms.