The "default" speakers in your car should be bi-wired ones. The 6-speaker layout in your car should have 2 speakers in the front, 2 tweeters in the front and 2 speakers in the back. The speakers in the back of your car should only have 2 strands each. The 4 strands of the connector on the front speakers are composed of two strands for LF - low frequency (the lower speaker in the door) and two strands for the HF - high frequency (tweeters in the dash/high in the door).
Disconnect the speaker you want to replace, then measure the ohmic impedance of all the strands. Two of them should have low impedance. These are the ones that go to the HF. You should leave those strands alone and connect the remaining two strands with your new speaker.
I don't know if the factory speaker has some circuit/filter for the HF signals though. I think its best to hook the new speakers up with the two strands that have very high impedance and leave the other two dangling. Then test your speakers with a song that has a lot of high tones in it and put your ear against the tweeter. You should hear some sound now. If not there is probably some circuit mounted in the default speaker.
Before you get started, you must measure the two new speakers on impedance. They must match and this value (i.e. 8 Ohm) should be roughly the same as the the impedance of the default speakers. Your audio system is balanced and will work best with speakers of the same impedance.
Hope this helps..