For starters, your 4 yr old is in a booster and HAS to be in a position with a shoulder belt...which most likely means on one of the sides.
I know the middle position is the safest position and while it may not be "easy" its probably easier to lift the carrier (assuming that's the kind of seat you're using for the newborn) into the middle and to have the 2 yr old on the end so you can help her get buckled.
Its not always possible to get a new vehicle, so you make your vehicle work as best as you can :)
As for the comments regarding the front seat:
In MOST situations there should never be a car seat - forward or rear facing in the front, however, some vehicles don't leave any choice - like a pick-up truck. Those vehicles had an airbag shut-off making it ok to put the car seat in the front passenger seat.