Mr Maillaud said: "We are waiting for the ballistic team's report and, when possible, a hearing with the eldest girl.
"Maybe she can give us information on the number of people present, for example, or the colour of their skin, and other elements of description that might allow us to consider a bit more seriously a first lead. Medical staff have not yet given us access to the girl, we are waiting for their decision."
Zainab was beaten repeatedly around the head and shot in the shoulder in the attack on the family from Claygate, Surrey, which took place on Wednesday. She was hit with "tremendous ferocity", said the prosecutor.
Her younger sister was found alive in the BMW estate underneath the bodies of her relatives about eight hours after the massacre, which also saw a passing French cyclist, Sylvain Mollier, 45, shot dead.
Mr Maillaud said he did not expect the four-year-old to be able to help police. He added: "She is a little girl who was hidden. It's not her who will help us progress the inquiry."