The trick is to use a 4:3 full screen resolution. E.g. output it to 1024 x 768 and you should have a nice 4:3 image.
On a widescreen LCD make sure your Nvidia or ATI drivers are set to not stretch the image...
However as Qbix mentioned it will look a bit "rough" on LCD monitors.
The only way I get an image that is:
- 4:3
- smooth looking
Is by using the "original" full screen resolution setting and letting the display adapter to the scaling. It will look nice, but games won't have the square pixels that DOSBox gives you with the inbuilt scaling unless you use a CRT or a LCD with a resolution that scales well (e.g. 1600 x 1200)