Interesting catch on his having fought him/her before. Now I'm going through to see if we've seen any other interesting people on his game screen while fighting.
It's hard to say how the rankings work. If it's like how chess works, it's entirely possible to displace someone who's in fourth place, without whoever is in fifth place being knocked out of their rank. The problem with this is that Tagiringer also played Kai. Making that scenario far less likely, as it decreases the chances that Tagiringer's rating would be low enough after beating Kai to be able to jump as high as it would by beating someone in the top four.
Another explanation that might fit the sequence of events is that when Kai was playing Tagiringer in episode 3, Tagiringer was ranked higher than fourth. So, when Kai sees Tagiringer ranked fourth in episode four, it comes as a shock. Though that's kind of a weak explanation as well, since you'd think he'd be more surprised at whoever was now in Tagiringer's old spot.
As you pointed out, Jan, there aren't any good suspects among the younger characters. I'm still not sure how much of Misa's personality is a facade.
What I am sure of is that it is a facade, to some extent. That comment of hers in episode 3 that seemed to concern the Committee was the closest thing to her real persona that we've seen so far, and that Misa most definitely didn't seem like the adult Misa we've seen throughout the rest of the series. As she was alone and speaking to herself, it's likely that one moment was the "real" Misa.
Also, Tagiringer is colored red and yellow, just like a certain robot Misa used to operate in real life.
Coincidence!? Ok, maybe it is....or maybe it isn't...
Arg. As I'm scanning through the previous episodes, I'm also noticing the Exoskeleton is actually a new product and just went on sale as stated by Misa on the TV in episode one. Yet, the shop keeper got hers 4? 5? years ago, which means hers must have been a beta/alpha product that Misa gave her. I wonder if that's suppose to mean something, while being fully aware it's entirely possible to read too much into things in a show like this. :/