Where to put the number in organic nomenclature?
The IUPAC recommendations on this matter have been changed.

According to the 1979 recommendations, the correct name was still 4-hydroxy-2-pentanone.

According to the 1993 recommendations, however, locants were placed immediately before the part of the name to which they relate. This rule is still included in the 2013 version. Therefore, the correct name is now 4-hydroxypentan-2-one.