Naming tetrahalogenated benzene
The locant set ‘1,2,4,6’ is not lower than ‘2,1,5,3’.

According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the lowest locant set is defined as follows:


  P-14.3.5 Lowest set of locants
  
  The lowest set of locants is defined as the set that, when compared term by term with other locant sets, each cited in order of increasing value, has the lowest term at the first point of difference; for example, the locant set ‘2,3,5,8’ is lower than ‘3,4,6,8’ and ‘2,4,5,7’.
  (…)


Especially note that the locants are cited in order of increasing value when they are compared.

Thus, the locant set ‘2,1,5,3’ is regarded as ‘1,2,3,5’, which is lower than ‘1,2,4,6’.

Therefore, the correct name is 2-bromo-1-chloro-5-fluoro-3-iodobenzene.