loc 2ch w/ adaptors or 4ch w/o?

Whoa, that's confusing.  Why did you buy a four-channel amp to use with only the rear two channels?

Basically, use a 2-channel LOC with 2 channels;  a 4-channel LOC with 4 channels.

I don't really understand what you're trying to do, but here is a scenario:  say you have a 4 channel LOC on hand.  But you only need to use it for two channels, like the two rear channels in your question.  In that case, use only two of the LOC's channels.

Now, if what you have on hand is a 2 channel LOC, use it as normal onto the two rear channels.  This will feed two channels of the amplifier.

If, for some reason, you want to feed this same signal to the other two channels of the 4 channel amplifier as well, use Y adapters to split the signal.  Impedance is determined by the speaker load on the amplifier, not by the signal source.  If this 4 channel amp has a speaker on each channel, its two connected right speakers will both play the deck's rear right output.  The impedances of channels 1 and 2 will not affect impedances of channels 3 and 4.  You could put 2 ohm loads onto 1 and 2, and 4 ohm loads onto 3 and 4 if you wish.  Those impedance loads are dictated by the speakers you choose to connect to the amplifier.

And, as I said, splitting the input signal does not affect impedance.  Does that answer your question?