Safety is considered. Twin-engined aircraft must operate to more onerous requirements than four-engined aircraft, so that the overall risk is considered acceptable. CASA do not say that 4 engines are safer than 2 engines, they say that 2 engines operated like this are considered as safe as 4 engines operated like that. They have made safety win over costs by insisting that the "less" safe operate differently. If they didn't care, twins could do what they want.