If I’m not a PlayStation Plus member but I log into a PS4 system owned by a PS Plus member, will I be able to play online multiplayer?
Yes. If you don’t have a PS Plus membership, you can still access online multiplayer modes if you log into a PS4 which is set as “Primary” by a PS Plus member. This means that, for example, if one housemate is a PlayStation Plus member, everyone in the household will have access to online multiplayer. Additionally, on the same PS4 system, any games or content purchased by one user are available to other users who play on that console (unless that content has expired).