Potassium hexacyanoferrate(II) has a complex polymeric structure made up of octahedral [Fe(CN)6]4 centers bonded together by K+ ions. It is a complex salt as well as a 

potassium hexacyanoferrate(II) is a bridging ligand (a ligand that bonds to two different metals, forming a bridge between them). 

Hexacyano ⇒ 6 molecules of CN. Hence, because CN is negatively charged, it wil also need 6 positively charged ions(metals) to bond with it. However, since ferrate is already said to be ferrate(II), it implies, 4 positive ions will be needed more, and that should be from potassium.

Thus the correct formula for potassium hexacyanoferrate(II) is K\(_4\)Fe(CN)\(_6\)