Learning language

I have earlier here talked about my learning of Portuguese, and the last two years have made wonders for my language skill. Since October of 2008 I have worked in Brazil, together with Brazilian crew (among other nationalities), with varying responsibility for external communication and instructing the crew. That have resulted that my Portuguese both for private use and professional use have improved radically.
It is a bit unimaginable to think that only 4 years ago I didn't speak a single word of Portuguese, there are still words I don't know, and I still have problems with the grammatical, I guess most Brazilians can hear I am a foreigner. There have been work situations where I start the communication in Portuguese, but end up speaking English as a deck foreman or crane operator tries to utilize his limited English skills.
My son also have had improving steps in his language learning. For some weeks now he have commanded "Vai la" ("go there") when he wanted something, something that reminds me of histories I have heard about my great grandfather who only looked over the table and commanded "send me that" when he wanted something during family dinners.
Today my son for the first time added what he wanted: "Vai la peta", "vai la dedei", at least he tells me what he wants now. I have read that two years of age children can combine sentences of two or three words, and my son definitely does that now. How long time does it take before he combines 4 words?
There have been some time since my last post, and my son will now soon become a big brother, but more about the new family member in a later post.