OTHER OPTIONS
I'm not sure if this mom is still checking the thread, but I woke up this morning remembering it & thought to add something.
I might be wrong in assuming that the infant is in the rear-center position with the 4 year old next to him/her {with/without a booster?} ... but, since my 6 year old stepdaughter {who wears size 8/10} only just outgrew the Marathon, I think the best solution would be to get such a seat for the older child & move the infant to the opposite outboard position ~ this way the older child will be less able to reach the infant to cause "trouble".
On the other hand, if there are other family issues mom should try her best to be very positive in her approach so that the older child doesn't rebel further & in the end form a lasting negative feeling about car safety AND a more lasting negative feeling toward the new baby. If mom could pleasantly convince the older child that the new seat is for his/her safety because mom loves him/her & is NOT a punishment, then all should be well.
The Marathon is a rather expensive seat, but would fit the older child & then be perfectly suitable for the baby when s/he outgrows the infant carrier
It's a valuable long-term investment that would help to end the "problem" at hand as well as keep both kids safer longer! {After the Marathon, the older child should move into a booster, though, regardless of whether or not s/he sits in the rear or front}.