The fourth safe that you can encounter as Abby in The Last Of Us Part 2 is found on Day 2 of the girl's adventure, more precisely in the chapter The Shortcut.
In particular, during your journey with Lev through the neighborhoods of Seattle you will find yourself on an elevated expressway, which stops abruptly near a couple of abandoned cars. Turning to the left you will notice a balcony of a residential building on which you can jump: do it and enter the apartment through the window. On a small table there is one blackboard, with some marker drawings depicting maps of the condominium and the names of the occupants of the apartments. On the same table there is a message which reports a conversation between Sam and Julia, two neighbors, in which the first refers to the safe code as the number of his apartment followed by that of Julia.
To find the combination you will have to go out on the landing of the floor and check the names on the bells to find the numbers you need, helping you with the maps drawn on the blackboard: in particular, Sam is reported as S. Howels and the number of his apartment is the 302, while Julia is J. Kennedy and her apartment is the 304. So the definitive code is 30-23-04. To find the safe, all you have to do is go back to the original apartment and enter the bedroom, to then collect the resources, ammunition and parts of weapons that you will find.
We remind you that on our pages you can consult the guide to find the combination of the camper safe, the first one you come across as Abby, and the guide to find the code of the pharmacy safe, the last along Ellie's path.