Big 4 articleship very very urgent plz help

One of my friends has recently written his IPCC Exams. He wants to make optimum utilisation of Articleship. So he is thinking of trying in Big4. But I heard that in Big 4, each article will be delegated to only one area (such as audit or income tax or indirect tax, etc) throughout his articleship period.  In that case it will not be of much use to him if he joins in a Big 4 firm. 

So please clarify whether it is true that articles will be delegated to only one area in big 4 throughout their articleship period.

Thanks in advance.