I am not clear about this arrangement. Say you are a labor supplier who supplies labour to company A; and you also use another outsourcing company C to supply labour to campany A. And you invoice A at C's payment+4% Is that the case?
Labour supply is liable to NBT and the question is the basis for NBT calculation. Theoretically it has to be the same basis that you use to charge VAT. If you recognize total amount (cost+4%) as sales and charge VAT on the total, then you re required to pay NBT also on that basis.