Oxidation state, also referred to as oxidation number, describes the degree of oxidation of an atom in a chemical compound.
Oxidation state of Vanadium in NH4VO2 is +3 .
Oxidation state of Vanadium in K4[V(CN)6] is +2 .
Oxidation state of Vanadium in VSO4 is +2 .
Oxidation state of Vanadium in VOSO4 is: (VOSO)4=(VO)2++SO42−x+(−2)=+2
Hence, x=+4