If you had nothing to do all day except play and were surrounded by adults speaking four different languages 24/7, you'd easily pick up all four to a basic child level within four years.

Languages aren't actually hard. They're only hard when you're trying to fit them into the many other demands an adult has on their time and because you're expected to have at least a minimal adult vocabulary, which is much broader than a child's.

The one element that gets harder with age is learning to control your mouth muscles to make unfamiliar phonemes. Learning to make new sounds with native fluency is harder at 40 than it is at 4.

If you want to do this properly you'll have to hire a voice coach who specialises in native intonation and can take you through a detailed physical retraining program. This isn't something most language courses/teachers can give you.

Full immersion works too, ideally with a native partner. But it takes longer and you'll usually still some have accent left.