The Japanese demo for SSB4 is live! Sakurai also announced on his daily Miiverse post that the US should be expecting a demo soon (stay tuned)!
Rumour has it that the demo is coming out on September 19th. The demo itself will have about 10-30 uses. Available characters: Mario, Link, Pikachu, MegaMan, and Villager. Only one stage, Battlefield. Both versions (Regular and Final Destination).
News via Nintendo Everything
Rumours via SmashBoards