If your 4 year old is really that small, it sounds like the Marathon might do him fine until the booster stage, correct? Which means you can purchase a suitable convertible seat for your new baby, and not worry about the Marathon being too large.
The issue with the Marathon is this--Britax had initially said yeah, 5lbs (I believe it was 5?) is the minimum. Then they decided to say that the child had to be a maximum of 1" from the bottom harness slots... For my son, way back when they said this (2004 I believe) that meant approximately 7 or 8 months old. Then more recently, Britax has been saying that the child should be at or above the harness slots when rear-facing, INCLUDING that bottom slot. I'm not sure if their most recent recommendation is completely reasonable for the convertibles that do not have infant inserts, since my kids didn't reach the bottom slots until they had long since outgrown their infant seats.
Anyways--I would recommend against using the Marathon from the newborn stage, regardless of the cited weight by Britax. The fact of the matter is that newborns do not tend to fit well in larger convertible seats, and based on what you've told us it's really not even necessary that you pass that seat along at this point. There are convertible seats that work well from birth--I believe people have had success with the Radian and the True Fit--and you will get a full life out of that seat with some smart shopping.
If you have a good newborn sized doll, or a friend's new baby to borrow, you should go look at some of the other convertible seats out there. If you want to avoid purchasing another seat, I would highly recommend getting a seat that has low harness slots initially, but higher top harness slots and high weight limits. Definitely make a note of the date of manufacture, ensuring that it is as new as possible so that you can keep your new baby harnessed as long as possible...and purchase the seat as close to your due date as possible (without going over, of course!)
-Nicole.