I've heard of people learning languages in that short of a time span, but I don't know how reasonable it is to expect that kind of success. http://www.alljapaneseallthetime.com/blog/about The above link is written by an English speaking person who claims to have learned Japanese in 18 months. Pretty interesting.

It depends I think rather on the person than the method. You can live in a country where they speak the language you want to learn, and still not learn much. It comes down to how much motivation you have. But living in a country where the lingo is spoken can be a great help by supplying a sense of urgency to learn the language. I for instance want to learn Spanish, but I'm making slow progress because I don't want it bad enough. So I ask people here to help me, instead of helping myself:P Anyway to go from absolut beginner to learn a language fluently in just 4 months I think is really difficult. However you can learn a great deal in 4 months but maybe not an entire language from scratch.

It depends. If you live in a country that speaks english, maybe, but if you live in another country, who persons don't speak english, some years.