Another 2c - if you have a problem in CDG and miss the EWR flight, AF has a non-stop to IAD at 4:20pm. AF also codeshares flights with DL, so if necessary they could put you on a DL connection of some sort. If you have problems in EWR, CO has multiple flights to IAD, they would just put you on the next one. In either case, it shouldn't matter who issued the ticket, your best move is to know your options so that if the agent doesn't offer what you know is possible, you can ask.