Did you just get the graphics card? Do you get a message saying that the game doesn't recognize the graphics card? I got a new graphics card and both Sims 3 and 4 popped up a message saying that it does not recognize the graphics card and would put the game on the highest settings possible. But I played fine after I clicked the tick. You could put these types of threads in the help section.
I'm assuming the same operation should work for TS4. The TS4 equivalent file is not called DeviceConfig.log, it is called GraphicsCard.sgr. I would strongly suggest do a bit more research online to confirm all this before you start altering anything. And certainly keep an original backup of the GraphicsCard.sgr to be on the safe side.
Bur before you start doing anything, The best confirmation to check is in the same Sims4 folder where the Mods folder is, there is a file called Config.log. Open that with Notepad and look down the list to "Graphics device info" section. See there if it specifies the name of your GPU and also if it says "(Found:1, Supported:1)" If yes, then it does support your GPU. But if any of them say 0 instead of 1, then you're going to have to do the manual file altering thing. Or, alternatively, go to the EA Help website and report their file deficiency and hope they fix it in a patch.
Edit - Just to add some helpful info. I opened the TS4 GraphicsCard.sgr just now, and it does list the GTX 960 as being supported. Is yours a straight up 960? Or something else with more bells and whistles like a 960m or 960msi?
Also, at the expense of pointing out the obvious just to be thorough, is your NVidia driver up to date?
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