The rumour I posted about at the weekend has now received official confirmation as well as a few additional details courtesy of Capcom's own website. Not only will KobaP be presenting new information and footage from Sengoku Basara 4 at the Butai Sengoku Basara stage play event on Saturday 13th July 2013, but Ikeda Shuuichi will also be making a guest appearance!
For anyone reading this who doesn't know, Ikeda Shuuichi is the voice actor who appears in the Sengoku Basara 4 teaser trailer currently running on the official website. He's said to be the voice of a new character - but not one of the two whose silhouettes appear in the trailer itself.
Note that the event, Busho Matsuri, lasts for two days (with two performances on the Saturday), but Ikeda will only be appearing on the 13th at the 12:30 event. We should start hearing online reports of what was announced a few hours after that time.
The Busho Matsuri website has also been updated with the new information. Tickets have allegedly been selling very quickly during the presale period so arrangements are being made available to accommodate more people who weren't able to buy an early ticket. This is great, since the presales took place before the Sengoku Basara 4 link was made and fans who don't follow the stage play might have missed out entirely.