Hi S., I just spoke with my husband who is a great paramedic and has been in the field for 15+ years. First he said that he would be worried as a parent too but that this is not that uncommon and is not necessarily a life long problem. He said that if the tests are coming back negative than the Dr's really have to look into some kind of viral condition that may be lurking in the background. A virus, I'm sure you know, will not show up in the bloodwork as bloodwork is aimed at bacterial infections. He also said that if the onset of the 1st fever/seizure started with a difficult teething time period then the 2nd, 3rd, and 4th seizures may be due to an "automatic" response your child may now be predisposed to following any fever even one that is not that high.
To ease your mind a bit, he did say that many, many children grow out of this. He said to push the Dr.'s for more information and tell them to look down avenues they don't ordinarily follow like the virus or the teething.
I hope this helps. I know first hand how scary a seizure appears. It is heartbreaking. But you are doing all the right things. DON'T give up hope. Keep remembering that this may just be a stage in her life and you will make it through whatever it turns out be.
Be Stong. Be brave.
H.