First of all, congratulations on the pending newborn!
There are lots of things going on here, so I will try to simplify
Firstly- Putting the 4-year old child back into a harnessed car seat {this would be much easier for your 3-across, especially since it will be tight! Figuring out booster seat that work in a 3-across is difficult because the child must be able to buckle their seat, which is hard to do with car seats on either side}. Is this option something you would consider? If not, we will try our best to find a booster to work for you but it may be very difficult!
Secondly- The Britax Roundabout that has a 40 pound weight limit (which would have been the one out when your oldest was an infant) is not suitable for a newborn. Even with low weight limits, the smallest harness height setting is too far above a newborn child's shoulders to provide a safe, secure fit around their body. They must use a seat with harness straps that come from at or below their shoulders. There are many, many choices out there for all budgets! What is the weight limit and date of manufacture of your Roundabout? This will help us figure out if it will be suitable.
Thirdly- As you cannot retrofit a top tether anchor in the center position of the rear seat, that seat should be considered only safe to use for a rear facing child restraint. Lap belts alone are not safe for anyone to use. This does not mean you have to buy a new car (at least not right away). If you follow the recommendations of the AAP, NHTSA, Transport Canada, and Child Passenger Safety Technicians/Advocates and rear-face your child to at least age 2, then the vehicle will work for your family until you turn your newest child forward facing. The longer you can rear face, the better
It is over 500% safer, and many children find it more comfortable!
Questions:
(A) What is your budget for a new seat for your newborn?
(B) If you are willing to re-harness the 4-year old, would you be willing to purchase a new seat for the child? If so, what is your budget for that seat?
(C) Are you opposed to skipping the removable infant carrier and going directly to a convertible car seat for your newborn? (Provided the seat fits the child properly it is equally as safe as an infant carrier)
I'm sure we can get you all sorted away and on the road with all three on board safely!