I've flown into Paris three times through CDG. Each time I was fortunate enough to route through Iceland (so passport control for Schengen was there and lines are non-existent) with only a small carry on. Not needing to stand in line for either luggage or passport, it consistently took me right at one hour to be on an RER leaving CDG (including getting to the train and buying a ticket). RER B takes right at one hour to get to Montparnasse from CDG (I'd rather use RER B because it isn't subject to street traffic).
You want to be at Montparnasse about fifteen to twenty minutes before your train, a little more if you need to get a ticket.
Four hours leaves you less than two hours for everything else at CDG. Is that enough to get your luggage and through passport control? I've heard horror stories so I wouldn't bank on it.
Personally, I'd plan for train times but buy my ticket when I got to the station. I've missed my train before and talked to someone dressed like a conductor at the station. It was no problem to catch the next train (but the next train had more stops than my planned train so lost nearly another hour). Some tickets are worthless if you miss your planned train and you'd be stuck buying a new ticket anyway.