Within both the Taronga and Melbourne herds the elephants, however cohesive do seem to spend time hanging around other individuals more than others Perhaps if all 4 females do end up having calves it might be more sensitive to the elephants to split the adults up and allow mothers and calves to stay together.
Forming strong friendship bonds seems typical of elephants in captivity(and in the wild?) Sometimes these bonds can arise between comparative strangers rather than longterm familiars too.
At Whipsnade, the four adult females form two 'pairs' of friends- I believe each pair comprises one Whipsnade and one ex ZSL cow, not the two original Whipsnade and the two from London as you might expect. It seems to demonstrate a distinct choice on the individual elephants' part.
At Chester the newly aquired female 'Birma' which had previously lived alone in a French zoo, formed a strong attatchment to their old matriarch 'Sheba.'
EEP recommendations are to move mother/daughter dyads to form new groups so that female calves can stay with their relatives long term. This can't always be adhered to for practical reasons though e.g. the young female which was moved from Twycross to Whipsnade.