Navigate to SWAT 4ContentExpansionSystem and open Swat4X.ini in a text editor such as Notepad.
Search for the following lines:
FullscreenViewportX=
FullscreenViewportY=
Modify these lines to suit your resolution. X is horizontal and Y is vertical.
The following example would force the game to run in the resolution of 1440x900.
FullscreenViewportX=1440
FullscreenViewportY=900
After changing the values, save the file and make it read only. This will prevent the game reverting back to a 4:3 resolution.
When launching and exiting the game you will get the error "File Not Saved: Failed to write SWAT 4ContentExpansionSystemSwat4X.ini." This error is fine, just click OK to continue.
Some vertical view is lost when running in a widescreen resolution.
Screenshots (click on thumbnails for larger images):
4:3
16:10