I've always put a small sub in cabinets I've built just to fill out the bottom end a little. I've also tried to acoustically couple the subwoofer to the cabinet as much as I can, either by laying it on the longest edge and weighing it down, or just bolting it into the body.
To me it's felt like you can have a more full sound at a lower overall volume when the bottom end is being filled out, even if it's just a 4" sub.