[CentOS] Extract lines from text file

Hi,

I'm looking for a command to extract lines from a large text file, a 
password file. A typical user has a username made from a letter 
followed by their id-number.

m9718508:pw:9301:840: Lynch  :/home/pgstud/m9718508:/bin/bash

So for instance if I need to extract lines where;

the 1st field, the username begins with an m
and
the 4th field, the group contains exactly 850

cat passwdfile | grep ^m | grep 850 > output

is close but fails if the value 850 appears outside the 4th field. In 
the above example which should be ignored 850 appears in the username 
and home directory and is therefore extracted.

Any ideas.

Thanks,

Tony