There are very good number of C.A firms in Mumbai. If you are talking of big 4 then it is very difficult to get admission until and unless you have very good result and you know some one very closely. If you feel you can try
4) Pricewater House
5) Khemji Bhimji
If you really want to be successful C. A. then do not go for very big firms find out the place where you want to do your Articleship. It is important to find out a firm near your residence to save time in travelling in Mumbai.