Which are the correct IUPAC names of the following compound?

1. 5-Butyl-4-isoporpyldecane

2. 5-Ethyl-4-propyldecane

3. 5-sec-Butyl -4- iso-propyldecane

4. 4-(1-methylethyl) - 5 - (1-methylpropyl)-decane

(3, 4)
5-sec-Butyl-4 iso-propyldecane
Although IUPAC name for sec- butyl and isopropyl groups are 1 methyl propyl and 1-methylethyl respectively yet both these names, are also recommended for IUPAC nomenclature.