The given compound has longest chain of 10 carbon and substituents are on 4^th and 5^th carbon. On C4 there is isopropyl group also called as 1- methylethyl and on C5 there is secondary butyl group which is also called as 1- methylpropyl group.

Therefore, the correct IUPAC name of compound will be- (iii) 5– sec-Butyl – 4– iso-propyldecane (iv) 4–(1-methylethyl)– 5 – (1-methylpropyl)-decane