The fluency achieved in even four years is very very limited: very correct from the point of view of how well the grammar used by adults work (better than adults over that period), but the range of the things they can express is much smaller than adults learning the language over those four years (several hours per day).
To add to this, the vocabulary and the types of sentences that children will construct are much simpler. They may talk about animals, what they've done at school and what to play next. An adult reading a foreign newspaper, by contrast, has to be able to understand language around current affairs, politics and socio-economic events and structures. A very different side to any language.
Absolutely. I have lived in places where the main language is not in the family of those you speak and I have experienced two situations: a) you have to work every day and there is little to zero chance of being exposed to the language at work, plus, little over the weekends, no possibility of finding courses outside working hours, etc. and b) spending the first months six hours a day exclusively learning the language. Well, in my unscientific experience, with b), in months you get the fluency that adults typically get in about two years.
I can absolutely believe that. I'm a relatively recent transplant to a new country and though I fall into your category A and am nowhere near fluent, I could conduct basic daily affairs (ordering at restaurants, pay bills at a post office, make basic appointments) in about 2 months or so of being here. I think adults don't give themselves enough credit in language learning.
