Although The Sims 4 doesn't require an Internet connection, it's needed to activate the game and start the game. Unlike The Sims 3, The Sims 4 won't start if you're not connected to the Internet as it has to sign in to Origin. Likewise, The Sims FreePlay will not work at all without an Internet connection.
The results of when I start The Sims 4.
- Connected to Internet & signed in to Origin: Can play The Sims 4.
- Disconnected from the Internet but signed in to Origin: Origin automatically runs in offline mode and can start The Sims 4.
- Not connected to the Internet & not signed in to Origin: The Sims 4 won't start as Origin won't connect.
I know that in Steam, if I have problems with Steam not connecting or I'm not connected to the Internet, I have the option of running it on Offline Mode instead so I can play games offline if the account was saved. This isn't the case with Origin. When I tried to access my account, Origin won't let me use my account offline so therefore I can't start The Sims 4 if I'm not connected to the Internet. If I was signed to Origin but got disconnected, I'm still on the Origin account and can play games offline. Note that I do Origin to remember me when I start it so I don't have to sign in every time I start it.
So it seems that to me that it's impossible to start The Sims 4 without being connected to the Internet as it fails to sign in to Origin. I know Origin has an offline mode but without an Internet connection, I can't use my account. I'm not sure if there's a way to start TS4 without an Internet connection.