I am thinking that if character DLC happens for Smash 4 that there is a good chance that the DLC characters will be "on disc". There are two primary reasons for this:
1. Namco already has done this before with Tekken Tag Tournament 2 and Street Fighter x Tekken
2. The limited internal space for the 3DS.
Wait...what? Limited internal space? What are you talking about? There isn't really a limit for 3DS downloads. HUGE games like Resident Evil: Revelations are available to download from the eShop, and I'm pretty sure there hasn't been any "on-cartridge" 3DS DLC. Theatrhythm Final Fantasy, New Super Mario Bros. 2, Sparkle Snapshots 3D, Art Academy: Lessons for Everyone!, Style Savvy: Trendsetters, Fire Emblem Awakening, Sonic & All-Stars Racing Transformed, Pokémon Mystery Dungeon: Gates to Infinity... None of the DLC from any of those games is all on the cartridge. And Nintendo doesn't do DLC that way. It doesn't matter what Namco Bandai has done before. The game's being published by Nintendo, and there's no way Nintendo would allow on-disc or on-cartridge DLC like that when they've said that they're against it.