Hi Never messed with these so just guessing really but the reason for the four wires instead of two is more than likely due to the tweeters built into the rear door. If you want to disconnect the tweeters in the door then connect the stereo and turn it on. Use the one wire from one side to determine which one powers the speaker and which one powers the tweeter. Then repeat the procedure for the other two wires. Then you will have the two wires you need for your new speakers.
Hope this helps
Cheers Dean