zbug112 Does Options.ini still list fullscreen as 0 (zero), or did the setting revert? If it reverted, try imposing the setting again and see what happens. If it still reverts, do you use any app that is supposed to "optimize" your game settings, or anything along those lines? It may be interfering here, so please disable or uninstall the app, or at least prevent it from touching Sims 4's settings.
You can also try forcing windowed mode in the App. Click Sims 4 > Manage > View properties, and add -w in the command line box.