There isn't a 4way speaker on the market that isnt junk and there's no difference in bass reproduction in 2way, 3way, 4way speakers of the same size/manafacturer.
2way coaxial (tweeter permanently mounted to midbass) or 2way component set (seperate tweeter and midbass driver and a crossover network) are the way to go.
If you are keeping the factory head unit and staying within a budget then a pair of coaxials for the front and another for the rear are sufficient.
You could go with components in the front if you like but dont waste any money on the rears, in fact you would be just fine keeping the factory rears intact and spending the money saved on better front stage speakers.
I'm waiting for my 2011 to be delivered in Feb and will be dumping $4k+ in audio gear in it (I'm not a basshead, I'm into SQ and drive all day every day) If you need any more help let me know, I've been out of the loop for a few years but have been doing this for 30 years.