There are two ways to solve your problem.
The first way is to check your monitor settings. Some monitors have the option to set the format to 4:3 or 16:9 depending on what you want to view. It is usually set to automatic so that whatever you are viewing fills the entire screen. However, this option depends o the type of video connector you are using (HDMI, DP, VGA).
The second and most easy way is through the NVIDIA control panel (I see you have a GTX 960). Within this control panel, go to the
Display section, and within the Display section, go to the section that says
Adjust desktop size and position. There you will find several options for how to display games that only have 4:3 resolutions, or even resolutions lower than your monitor's resolution. Here is a
video that can be helpful.