If the speaker opening are really 4", there is no way you're going to find a 3-way -- the speakers just aren't large enough to allow room for a midrange and tweeter.
If you're going to install an amp, a pair of Infinity Kappa 4" two-ways should work well for you:
http://www.infinitysystems.com/car/...EL&ser=KAP&Language=ENG&Country=US&Region=USA
I've always been a fan of Infinity's stuff, and while there are some that love to talk trash about them, those people are also the ones that usually buy uber-expensive, brands-you've-never-heard-of speakers.
Keep in mind that you won't get much by way of bass with anything smaller than 6.5" speakers, so for a nice, full-bodied sound, you'll want to install a subwoofer as well.
Also: 3-way speakers don't necessarily sound better than 2-ways. A good 2-way with a properly designed crossover can sound the same as, or better than, a 3-way. Also, if you have a sub, you can install a high-pass filter on your 2-ways to have them only play what the sub can't, and then you'll effectively have a 3-way system. It's much easier to properly tune and balance a system if you don't have multiple speakers overlapping in frequency range.