if you are referring to this one;
http://www.icsbb.com/news/e_news_main.asp?id=44
As an ICS M4 owner I would urge you to consider a G&P jungle series - and not a RIS model either.
For two reasons, the g&p is all metal, even the barrel; and they connect direct to the delta ring collar area of the body and the barrel as well. This makes for a rock solid connection when done right.
It takes a little filing/dremeling, but it's worth it and you'll never have to worry about it coming off.