yes you can power all your speakers using a 4 channel amp. I recommend that you install a "passive filter" or "passive crossover" of some kind to separate the frequencies going to the mids and tweeters (or 2.5" speakers).
A cheap way to do it is install a non-polarized capacitor of a correct value in series on the + line of the speaker if it doesn't have it already. Your other option is buy a more advanced passive filter or crossover.
Have you taken the speakers out yet? I can't really recommend a crossover point, but if I knew what the speakers looked like or maybe it has a cap installed already, then we could help you more. Or you could always just replace your speakers if it's too confusing with a "component set"