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There is a slot in the frame that you can fit a special hook thing into. The hook is common among tow truck drivers. I don't know what they call it but you fit it in and rotate it 90 degrees and it holds. I have a long chain with a J hook and this other hook on one ende, got it from a tow truck driver. There are also fittings coming out of the frame that you can get a hook into, behind the front wheels and in front of the rear wheels I believe, if memory serves me correctly. Good Luck.
On the fronts, I use race straps that have the hooks and hook them into the frame slot. The slots are right behind the front wheels. Some of them are harder to get to "hook" than others. Or I have a set of axle straps that I just put around the lower A-arms.
For the backs, I use axle straps and wrap them around the webbing of the differential cover. I do not have my spare tire cover in which makes tieing it down this way a breeze. With the cover in the way, it is more difficult to get to.
I've towwed my C4 for a few years now and I found that the four factory Tie-down loops that are part of the frame is the safest & strongest way to tie down your C4. They look like 2.5" or 3" D rings on the inside corner of the frame rail. I have four Tie-down points on my trailer at the corners and I cross the two front and the two backs. That way it holds the car from moving side to side & front to back. Also by using the car's frame it will compress the suspension, keeping your car from bouncing and unsettling the trailer during towing. RACERSWHOLESALE.COM sells them rather cheaply, Aprox. $20 a piece. Just as the other people have told you the front are right behind the wheels on the inside of the frame rail as bends inwards. The backs are right in front of the rear wheels. Put a little Yellow or orange paint on them so in low light conditions thay are easier to spot when your under the car at the end of the day. Good luck.