If you want to amp all four speakers and have the option of powering a sub too, Alpine makes a nice multichannel amp...here's their five-channel model:

http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WDVW

It's got 50 watts per channel and a 200 watt fifth channel for a sub, all in one.

I recommend it since I've been using the Alpine MRV-F340 4-channel amp for a year now, and it is outstanding for the price (I found it around $200 shipped on eBay last year). It is 55 watts per channel at 4 ohms (65 watts at 2 ohms)and sounds great, never distorts. You could also run two speakers with this amp and bridge the other two channels for a sub in the future...Best of luck with your project!