aggressive rat behavior?

Question
I have 4 female rats, 3 are kept in a large ferrit cage together, and the fourth is still pretty young so I have her in a separate cage.  I'm trying to slowly socialize the baby to the other three in hopes of putting them all together once the baby can't squeeze through the bars of the big cage.  The baby gets along with two of the three rats, however my hooded rat won't even give her a chance.  The first time they sniffed and then the hooded bit the baby under the jaw.  The second time (a few months later) she didn't allow any sniffing and went straight for her, biting her on the back of the neck. Both times causing a hole in the skin.  I've tried having them meet in different places, the first was a newly cleaned 20gal tank, the second was in the ferret cage.  I've also tried letting the hooded spend time in the baby's cage alone, and vice versa to get used to the scent.  Is there anything I can do to help the situation?? Or will the two of them always have to be separated??
I appreciate your time and look forward to your answer.
Thank you,
Natalie

Answer
Dear Natalie,
thank you for your question.
Sometimes introducing rats takes a very long time. Occasionally, babies are not accepted until they are older than two months, so that might be the problem in your case.
Put their cages next to each other, about 4" apart so that they can see and smell but not reach each other. Swap some of their bedding and some toys to make them get used to each others smell. If they don't bristle and hiss when they meet each other through the cage bars, you can try to let them meet on neutral ground with plenty of room, for example in the bathtub. A big hiding place with several doors (maybe a cardboard box) can be offered. Unless they hurt each other, just let them be, even if it looks dangerous. You may have to try this several times.
The getting to know each other-stage in two different cages can take long, sometimes months.
I hope I was of some help to you
Jennifer