What is the IUPAC name of K4[Fe(CN)6]?
All three would appear accepted by IUPAC, though A is preferred generally. 

Quoting from the red book (the IUPAC guide for nomenclature of inorganic compounds):

IR.5.4.2.2:


  It is possible to provide information on the proportions of the constituents in names by using one of two other devices: the charge number, which designates ionic charge, and the oxidation number, which designates oxidation state. In nomenclature, the use of the charge number is preferred as the determination of the oxidation number is sometimes ambiguous and subjective. It is advisable to use oxidation numbers only when there is no uncertainty about their assignment.
  The charge number is a number whose magnitude is the ionic charge. It is written in parentheses immediately after the name of an ion, without a space. The charge is written in arabic numerals, followed by the sign of the charge. Note that unity is always indicated, unlike in superscript charge designations (which are used in formulae). No charge number is used after the name of a neutral species.
  
  The oxidation number (see Sections IR-4.6.1 and IR-9.1.2.8) of an element is indicated by a Roman numeral placed in parentheses immediately following the name (modified by the ending ‘ate’ if necessary) of the element to which it refers. The oxidation number may be positive, negative or zero (represented by the numeral 0). An oxidation number is always non-negative unless the minus sign is explicitly used (the positive sign is never used). Non-integral oxidation numbers are not used for nomenclature purposes.


Potassium hexacyanidoferrate(II) and potassium hexacyanidoferrate(4–) are specifically given as examples based on this rule, making A and B both valid IUPAC names. 

Section IR.5.2.1:


  The proportions of the constituents, be they monoatomic or polyatomic, may be indicated in generalized stoichiometric names by multiplicative prefixes, as was the case for the constituents of binary 


Tetrapotassium hexacyanidoferrate is given as an example based on this rule, making C also a valid IUPAC name. 



As an aside, the red book specifically makes provision for the fact that, at present, there are multiple allowed IUPAC names:


  A planned future project on inorganic PINs will need to face the problem of choice between the equally valid nomenclature systems currently in use.


This differs from organic nomenclature when one preferred IUPAC name is given (known as the PIN), even where multiple valid names may exist (and are often preferred in everyday use for historical reasons).