Its not difficult to get accepted by one of the big fours in Hong Kong to begin with. They do not have enough people of all grades, as you may have been told, the staff turnover rate is so high that it is just so common to see three goodbye emails sent out by the quitters in a week. Unless you are very sure about the way you want to go and that you are already certain about the exceptionally long hours of work that you are going to get, you are very welcome to try. You should be reminded that there are so many other ways to get the three-year CPA experience besides the big four or second tier firms.
Also, in order to get your first year experience recognised in a big four firm, you have to stay there for at least ONE full year, that is, if you are sick and tired of working in such environment in the 6th month, you are expected to give up the six-month experience from your hardwork. As you may notice, I am one of the kind who has got stuck here in one of the big firms and quitting is not as easy as you finding a new job as a fresh grad. So take the advice and think before you act.