Couch co-op vs Couch co-op?!

Yup, totally doable.  But you really ought to make PS sub accounts and EA accounts for the ladies.  I'm not sure if you can even do this without them having their own accounts.  You'll both get your split screen games going and then one host (let's say you) will send an invite to both of the other players (your friend and his wife).

There are a few limitations when using split screen or playing in someone else's yard.  You cannot play the solo campaigns or go through the gnome portal in the sewers, and you have to be in your own back yard to promote characters.

Makes sense. But you can do 2 vs 2, or all four of us players vs a.i. horde mode, right?

Yeah, I had thought about getting my wife her own PSN account, but that's another $60/yr with very little (if any) benefit. We only have one TV and one PS4 anyhow.

Maybe I'll get this as gift for our long distance friends, so we can all game together.

Thank you!

You don't need another paid online account, she just needs her own gamer profile on the console rather than playing as guest.  You only need one PS plus account per console.  When she makes her EA account, make sure it's the same gamer tag and email address as the PS profile she makes.  It's pretty straightforward but I can't give quick instructions because it's been quite a while since I did it.

You can all four play ops or any multiplayer mode together.  You can play the multiplayer modes online against other players or a "private" game against AI's.  Each player can choose to be a plant or a zombie, so you could all play together against an AI team or split up on to opposite teams.  Or you can play a multiplayer mode with just the four of you, but generally speaking four players aren't enough to make those game modes worthwhile.  You can also just play around together in the back yard as any combination of plants or zombies you wish.

Create PS4 account:

https://www.playstation.com/en-gb/get-help/help-library/my-account/access-and-details/how-to-use-playstation-4--to-create-a-sen-account/

Create EA account:

https://profile.ea.com/