This rumor originates from VideoGamer which received an official PlayStation 4 brochure. According to them, the page regarding Infamous: Second Son on the brochure states the game will support a single player offline, with “online players TBC”. Games other than inFamous, such as Knack do not mention online players since they are to be played offline only.
The previous inFamous games were single player only, with the second installment having the option to create levels that could be uploaded online and played by other players.
Sucker Punch has not given us any information regarding this. If the game is to have multiplayer, we will know in the near future.
inFamous: Second Son is to be released exclusively on the PS4 in early 2014.