In short, yes. PlayStation Plus is required to play the vast majority of games online on the PS4. However Sony have stated that it will be at the developers discretion as to whether or not the player needs to have a PS Plus membership.
Free to play games supposedly do not require Plus membership to be played online, but so far only two developers have confirmed that their games do not need players to be a member. They are DC Universe Online and Planetside 2.
Now of course if you were to purchase a PS Plus membership or maybe you don’t even have too because your PS3/PS Vita membership will transfer over, you will sweep up the benefits that Sony have laid out for the PS4.
Some publishers have not confirmed whether their F2P games will require PS Plus, nevertheless here are the games that you will definitely be able to play for free should you have a membership:
Free for PS Plus members