I don't know what your current audio setup is your car. I think beyond a 4" would require some simple cutting with a dremel tool or something similar. I would suggest you take your door panel off and look with your own eyes, this will give you a better idea of what you are comfortable doing.
I bought 5 1/4" MB quart premium components off ebay for the front of my car. They were too deep to clear my window, so I had to put wood rings in to clear, then i ran into clearence issues with the door panel. I got it fugured out with 1/4" to spare on each side. They are great speakers just make sure you know what may have to be done.
As far as my setup goes.I would probably put an amp under my drivers seat. The inner sheet metal would need to be cut out to accommidate the larger diameter, A lot of sound deadning would need to be applied and the door would have to be completly sealed to get a good bass sound. I've seen it done on a forester. I'm not sure this task is beyond my ability, but i'm sure a custom audio shop would have no problems taking care of it if i messed it up. It's not on top on my to do list but something for the future as it could get a little pricey.