You'll need a PlayStation Plus subscription for online multiplayer on PS4. Sony revealed the change at its E3 press conference today as somewhat of a footnote between used game support and lack of online requirements.
Current PS Plus memberships will carry over from PS3 and Vita to PS4, Sony said, and it didn't mention any new pricing models. A one-year membership costs $50 at the moment.
Sony plans to continue offering free games, discounts, and other bonuses on top of PS Plus, but it looks the valuable option just became a bit less optional.