More or less a random thought really, but I could see Tokugawa being the original No. 4 of the 1st String.
I guess the first assumption to make here is that they didn't ditch a random guy somewhere in Korea, but that it was really only 18 players leaving for the exhibition matches. While Kurobe originally said it had been 20, we already know that this is false after Tanegashima was revealed as No. 2.
Now with that out of the way, the outstanding HSers in the camp were Tanegashima, Tokugawa, Oni and Irie. Tanegashima was revealed as No. 2, Oni as No. 5 and Irie was said to have a spot by Akiba, leaving only Tokugawa.
One thing to note here is that Oni apparently resigned, which resulted in Kaji taking his place. Tanegashima on the other hand simply didn't go and they left his spot empty.
We know that Irie, should he have had a spot, also resigned, so if he was the No. 4, he should have been replaced as well (i.e. it's not him). Tokugawa on the other hand is clearly displayed as strong, yet we haven't really gotten an explanation for why he isn't a 1st Stringer, aside from possibly that he didn't get the No. 1 spot and was salty.
There's also still the thing I brought up a few weeks ago that he (and Irie) are displayed with the rest of the top 10 before the start of the current matches.
That's about it I guess. I suppose it's somewhat optimistic to expect an explanation for the original No. 4, but if we get one, I wouldn't be surprised if it's Kazuya.
Could still easily have been Irie, and nobody replaced him.
Nobody replaced Tanegashima when they went.
So No.4 didnt have to be replaced if Irie Kanata didn't wish to travel.
Wouldn't be surprised if it was either Irie or Tokugawa.
Fayte's theory of Ryoga killing the ex No. 4 in a tennis match is still better though lol.
It'll be surely explained in the future.
This implies the ability was there.
And Akiba was simply arrogant thinking his month of scientific training was enough to overtake him.
Oni says he stayed to strengthen lower courts, but it doesn't rule out the possibility he was replaced.
And of all people to replace him, why would Tokugawa do it, who is clearly being a spoiled brat crying over not being the No.1 of the camp and throwing a tantrum by refusing to accept any other position.
(I believe that's what he has done since let's face it. From what we have seen, he would destroy No.7 downwards of the HSers)
Although the altercation between Ryoga saving Tokugawa might hint to something.
However there has been no interaction from anybody of the 1st String with Tokugawa besides Byoudouin laughing at him.
Also, I meant to say ''It doesn't rule out the possibility Irie wasn't replaced.
Also, Irie could have just decided to stay like Tanegashima.
I just don't see enough yet to imply Tokugawa was the former teammate of those G10 guys like Oni was.
To summarize the debate above, the original number 4 could have either been Kazuya or Irie.