who did the guy up front cause the chain reaction? If he broke suddenly and everyone else rammed him from behind then he's not at fault as they were too close to him. My ex-gf's dad got caught in a chain reaction like that, and the guy up front wasn't regarded to be at fault.
But back on topic. What did the damage assessors say? Because from their experience they should be able to determine who hit who first. And also car #4 would always feel 2 hits (I'm going on your side for this explanation) because the first hit would be from you, followed by the second hit to car #3. The question is, did she feel a third hit which is from car #6 which you believe is at fault. Using logic and clear thinking, I'm sure that you'll be alright. Make sure you keep us informed.