Hi E.,
I have both the Britax Marathon (for my 3 year old son) and the Roundabout (for my 18 month old daughter) in a VW Passat wagon. We opted to buy the Marathon for my son because it is one of only a very few carseats that have a 5 point harness up to 65 pounds (most are only to 40 pounds), the safest way for your child travel. We passed his Roundabout to my daughter. We absolutely love both Britax seats, and they install well in the Passat. The Marathon is especially easy to install forward facing. However, since your baby is only 4 months old, he will still need to be rear facing. The Roundabout fits rear facing in the Passat well, there is plenty of room for the front seats to be pushed back. While we never installed the Marathon rear facing in our Passat I would like to mention that it is MUCH taller than the Roundabout, and might be tight on space rear facing. I believe that both seats are very equal in width and have a similar footprint, the difference is in the height. If you install it in the middle of the back, that might leave enough room for your other children, but would be a little tight on elbow room.
My advice would be to go with the Marathon over the Roundabout, as you will get more use from it long term, I think it would be the better investment. By the time your baby outgrows the Marathon he will be able to travel without a booster seat (6 years old/60 pounds is the current law, I believe), and that will save you from having to buy a booster seat, which you will have to do with the Roundabout.
Sorry for being so long-winded, I hope this helps!
S.