Anyhoo back on topic, Japan has a long a great history of warriors. All foreigners must prove they are the best of the best by participating a duel of combat. How do you know you are being challenged? The most typical way is to be greeted by a bow, ignoring a challenge is considered extremely offensive. Challenges may come anywhere at any time, be prepared for multiple challengers.
helpful edit - takodanobaba is a must visit imo. the train station is flanked by two rather famous arcades. mi-ka-do is long known for hosting tournaments for niche fighting games and has active 3rd and GG communities. never been big for SF4 tho because of big box, the other arcade in baba. I actually haven’t visited big box since 2009 so not sure if it still gets much action but back then it was probably the hottest spot for SF4.
and baba in general is a cool area, as well as being a stop on the jr yamanote line.
If you are in Akihabara, Club Sega, Hey, and Taitio arcades are within walking distance of each other. You’ll pass them all walking up and down the street so you don’t need a guide or to remember the locations before hand. When I was there a few months ago everyone seemed to be playing the 3-d arena fighters like J-stars & Dragon Ball Xenoverse style. Some Gundam game seemed to be the most popular “fighting” game people played. There were a bunch of card based games that seemed to be popular too but I didn’t try any.
The 2-d fighter floor of Club Sega was virtually empty. I did see people lined up to play Street Fighter 2 but I can’t remember which arcade that was. I think each play was only 10 yen or something. I did see a decent amount of people scattered around different arcades but there were plenty of empty machines around. I don’t really have any Street Fighter specific advice but the business day in Akihabra seems to be Sunday so that might be when the heavy hitters show up.