Easy to do. Just make sure you buy a free air subwoofer that's ment to work in such an application. It uses your whole trunk as the box. You don't need a lot of amp power to make these work either.
I know a few people that have done four 8", or four 10" and even three 12" if you have the rear deck space. These applications can get stupid loud too.
Current : 2013 Mini Countryman JCW, 2007 Z4M Roadster
Past: '88 E30 M3, '69 BMW 2002 Ti, 2000 328i, Audi A6 TT, BMW X5 3.0i , Audi A4, Mini Cooper Clubman S 1.8T