It is true that the plasticity of children's brains is higher than that of adults. Research also suggests that in very early childhood, children have an exceptionally good ability to discern different sounds and the ability to learn how to make those sounds, although this ability is lost at some point.

However, I believe many people overestimate the advantage children have in learning a language. For example, a four-year-old can have a basic conversation and communicate their needs, but they are not yet able to have a complex conversation. Their vocabulary and pronunciation may not be fully developed yet.

It's important to note that this child has spent a significant portion of their life learning the language and is exposed to it daily, and likely has parents or others to practice their language skills with. Despite this, their language level is not yet perfect.

The point I am trying to make is that it takes a significant amount of time and effort to learn a language, and there is no shortcut or "hack." Most adults would likely do comparably well if they dedicated the same time and effort to learning a language as children do.