I agree, if they are fighting during introductions at any time, continue pouch
swapping, I did swaps every 3 days about, trying to keep the scents fresh.
this is not a race. Also you should try neutral unscented introductions,
like a bathtub. Then clean a cage, if all goes well put them in the clean cage.
I think for 4 the cage should be large, and once all 4 are happy in tub, spend next few night near the cage and monitor their behavior.
It could be the older one starts to feel they are on her territory and its time
to defend the territory. hard to be sure.