Speaking four languages by the age of 4 is just ordinary development, assuming you're in an area where all four are spoken. You should be speaking them before the age of 2.

"Speaking them". 2 years old are not fluent in any language. Sure, they speak but they vocabulary is extremely limited they can't even make themselves understood most of the time which ends up in them being incredibly frustrated.

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.

I have a 2 year old and I speak with her in one language the mom in another and me and the mom speak in a third language between us.

She knows words in the 3 languages and can construct phrases in 1 or 2 of those languages.

She is is also much more developed verbally than any of the kids her age we meet regularly but she definitely doesn't speak 3 languages.