Does your 4 year old still fit with her shoulders below the top harness position? A forward facing harness is outgrown in height when the child's shoulders go above the top harness slot, so just keep that in mind.
The Chicco Keyfit would be my first pick - it's on the narrower side, and it also fits preemies and small newborns exceptionally well, which is certainly a good possibility with multiples. I'm not sure you're going to be able to fit any two infant seats with the Roundabout though - you're probably going to have to replace it as well. I would get a Diono Radian for her, especially since you'll probably want at least one when the twins move to convertible seats anyway. One FF Radian with two Keyfits will definitely fit your vehicle. Then in a year you can do a second Radian, put both twins RF in Radians and move your oldest to a narrow high back booster like a Parkway, or a combination seat like a Graco Nautilus or Britax Frontier.