5-ethyl-4-methyloctane. Why can't we name this compound as
Practically, it makes no difference. Any chemist would know what molecule you're referring to. Technically (and I mean that in the worst possible way) the correct name according to IUPAC rules is 4-ethyl-5-methyloctane, because "ethyl" comes before "methyl" alphabetically, so "ethyl" gets a lower number assigned to it than "methyl".
Separate names with a comma.