And that brings up so many questions like who was the fourth-ranked since AB reveals Misaki becoming Level 5 when she entered Tokiwadai too and she never mentions being fourth once or was never implied to be yet maybe
"Because the standard for measurement is the worth of the ability to the ongoing research, you're #3 and I'm #4?" Mugino told this to Misaka in Railgun Chapter 30.
Three years ago, Mugino was 3rd Level 5. She may felt inferior to Mikoto because Mugino was kicked out of 3rd Level 5.
Also Shokuhou Misaki became Level 5 at least two years prior to current timeline. She is 2nd-year of Tokiwadai Middle School. So, three years ago, Accelerator was #1, Kakine was #2, and Mugino was #3. We don't know who was #4 and #5 Level 5 at that time.
keep in mind while i dont confirm this as a fact Mugino might have simply offed the unknown 4rd with Takitsubo and a bit of stealth(cant say a 5th had existed yet since Kimi was surprised there was seven level fives now)
Still i wonder if a dead 4th around those years will ever be revealed smh