Hi, I just wanted to first say that I'm really sorry you're having this problem with your girls. We have four yorkie girls named Mica, Maggie, Lindi Lou and Shayla from oldest to youngest.
We've had a very similar problem with our Lindi Lou attacking the other girls. She always had a very dominant personality even as a puppy which was one reason we really loved her because she is so outgoing. Our Mica has so many anxieties and is scared of her own shadow sometimes but was like this as a puppy when we got her. I think it was the difference in how she was socialized because we got her from a different show breeder than our other girls who were all very well socialized from the start.
I think as Lindi Lou got older she sensed Mica's lack of confidence and as such has tried to take over the alpha dog role in our household. Because Lindi Lou has a more dominant personality she has became more and more dominant as she got older. This could be why your youngest is starting to act out like this now too and especially if your oldest one is insecure and nervous like our Mica.
Even when we only had the three girls I found that Lindi Lou started to try to put the girls in their place when there was a scuffle on any kind or if the girls became hyper like when the doorbell would ring. Other things that would set it off is when one of the girls would yelp if they hurt themselves. Awhile back Maggie was yelping suddenly because she got stung by a bee and this set Lindi Lou off as well. From what I've read now, it's not that she's trying to hurt the other girls but in her own mind she attacks the girls at these times to restore stability to the pack which is still hard for me to understand. Otherwise she's the sweetest girl, turns over for tummy rubs and is the most friendly of all of my girls.
At the beginning I would separate them but after watching several episodes of the dog Whisperer with a similar situation I realized this was the worst thing I could have done. Cesar suggested leaving them together and turning the aggressor which was Lindi Lou on her side and holding the one she attacked right up beside her until she would calm down. I've been doing this for awhile now and these outbursts occur very infrequently now so I really think it's helped allot.
The reason not to separate them is that then the last thought in the dog's mind is the aggression that they last felt towards the other dog. This made perfect sense to me once I thought about it. If you leave them together then the one that has attacked after several minutes will calm down, and have a calm state of mind. Because of this they associate this feeling with the other dog that they just attacked when they are back together. I hope I explained this alright.
I've also learned to watch Lindi Lou's demeanor and can tell when she's starting to escalate into the "red zone" and stop the behavior before she would ever attack. I also know what situations set her off and will avoid these. The worse one is when the doorbell rings and they all run in a frenzy barking. I now pick Lindi Lou up right away and just calm her down and all is well with that.
I sure hope this gets more manageable and improves for you and your babies. I know first hand how stressful it can be.