One time when I was enjoying all that Laredo has to offer (well, ok, not ALL that it has to offer) I ran down to Best Buy to kill some time. Walked out with a pair of 4" Fossgate replacements, and had them installed in less than a half hour. Pop the pillars off, pop up the grilles, pry up the old speakers (pry hard on the edges, they are held in place with some unseen dorky clips) and plop in the replacements. Some people will file down or clip off the edges of the new speakers, where the screws go, to get them to fit exactly, but I found that when wedged just right, I could screw them down and the grilles would fit back in place with no problems.
Now, there's only so much you can get out of 4" speakers. The highs are definitely improved, and the midrange is slightly improved. The bass, of course, is the same, as in non-existent. One of these days I'll add some more speakers.