What actually you wanted to achieve with your 4 car speaker and Onkyo 508?
Use it for stereo or home theatre use or both? Home theatre set up is not achievable as you need minimum 6 drivers (one of them as a sub woofer).
For stereo use, you can use two car speakers (drivers actually) for two channels and use the other two for another room.
I request you to look at the manual of 508 and then decide. You will be in a better situation once you understand capabilities of the amplifier.
If you want to buy a stereo amplifier (available for 20K or less brand new) and use your car speakers, it is possbile with some ifs and buts. Again, you have to understand what those car speaker drivers want. For start, those drivers need an amplifier which can support 4 ohms impedence of a speaker. You alternatively can put two drievers in series (per channel) and use it almost any home use amplifiers. BUT, having two drivers driving the same frequency can add unintended colouration and distortion. I have not used car speakers at home but know people who had used and were happy with it.
It all depends on how much you know about your music system and how much you want from your system. Car speakers are designed to sound good in a small space, I doubt how good will it sound at home.
If you can, play those drivers in your home placing them bare on floor, listen to them carefully. If they sound really too lean, too harsh and too weak, forget them for home use. Remember, if they lack bass, it is not much difficult to gain it back, but if they lack mid, you will loose a lot. This is in consideration that you will listen them placing bare on the floor. Best is to let somebody hold it 3 feet away from any any wall or floor and facing you.
