you do or what should
accomplish this with a 2 channel amp (assuming 2 ohm stable). Run the Front and Rear Left speakers in parallel, and do the same with the Right speakers. This way, you will clearly lose the fader - which I personally wouldn't care for.
You could also run a 2-channel line coverter, and split the signal into two pairs, and use the gain on a 4-channel amp to fade the sound front to rear. This is OK - it just requires you to manually adjust the levels at the amp.
Or you could run a 4-channel line coverter (or, this could be the high-level inputs on the amp). Then, run a 4 channel amp. Once you set your gains, the 4 channels will work properly with the gain/fade controls on the head unit. This would be my choice, but I'm not paying the bill
I'm sure there are other solutions here as well...