Super Smash Bros. Brawl only got 35 characters due to time constraints. Sakurai did not actually say how many new characters he would put in for SSB4. I do not believe MarthFanatique on this. Random Fire Emblem characters are possibility, in the form of downloadable content. The most viable random (non-Lord) Fire Emblem characters include Sheeda, Oguma, Nabarl, Ayra, and Sothe. However, I may or may not expect either one at this time. I am expecting Lyndis, Isaac, and Little Mac. Marth and his sword will be redesigned after the recently released Fire Emblem: Shin Ankoku Ryu to Hikari no Ken. Fire Emblem is a financially emerging game franchise, so you may expect more than 3 representatives in SSB4, giving that Lyn as an Assist Trophy in Brawl counts to some extent. It may be financially small at the time of Brawl's release, but it has the potential to catch up with other Nintendo franchises by the time of SSB4. There will probably be only one new Fire Emblem Lord created between Brawl and SSB4. The next original Fire Emblem game may be created near the time of SSB4 and possibly on the same platform. I can (almost) assure you that Fire Emblem will sell better during the Brawl era (between SSBB and SSB4) than it did during the Melee era. Therefore, you cannot really predict how much Fire Emblem will be represented in SSB4. Brawl has also influenced sales of Fire Emblem: Radiant Dawn.
Sakurai also stated that adding only a few newcomers will not justify making a new Smash Bros. game. He is a big fan of Fire Emblem. SSB4 will feature voice chat, downloadable content, and other online features. You will have the option to turn voice chat on or off. Wii points may be charged for downloadable content (300 for characters, 500 for stages, 200 for Event Matches, Assist Trophies, and Pokéball Pokémon; and 100 for items, trophies, and music tracks.).