I found 10 Active MLB players who were apart of the 2004 Expos Organization but a couple of them aren't currently on the 40 Man Roster of an MLB Team.
Current MLB who played or were drafted by Montreal, but who didn't play for the 2004 Expos:
SP Bartolo Colon - 2001 Expos (plays for Oakland)
SP Cliff Lee- Phil (drafted by Montreal in 2000)
2B Brandon Phillips-Cin (drafted by Montreal in 1999)
MLB players who played for the 2004 Expos
SS Maicer Izturis (Currently plays for Toronto)
IF Jamey Carroll (Currently plays for Minnesota)
OF Endy Chavez (Currently plays for Seattle)
RP Luis Ayala (Currently plays for Atlanta)
RP Scott Downs (Currently plays for LA Angels)
RP Jon Rauch (Currently plays for Miami)
Players drafted/signed but who never played for 2004 Expos
OF Roger Bernadhina (signed as an amateur free agent in 2001)
SS Ian Desmond (this surprised me)
Players from 2004 Expos still play, but aren't on any team's 40 man roster)
RP Colin Balester (drafted in 2004, plays for AAA Round Rock- Texas Rangers)
SP Claudio Vargas (pitched for Expos in 2004, plays for AAA Buffalo- Toronto Blue Jays)