Feeding Growers to Chicks

My chicks are four and a half weeks old and are on Marriages chick crumb.  I've been following this thread about growers pellets and would like to know if it is essential to move them on to this soon.  I'm asking because I still have 3/4 of a sack of chick mash and only 4 chicks   there is no way they will eat all this for about another 4-6 weeks.

Also, I can't work out whether there is grit already mixed in to the feed, I've checked the web site but I'm still confused. Should I be feeding them grit as a supplement?

Thank you.