something is still missing

Juleous

You have a 4 channel amp, with that you could run front components off one channel set and then the rears off the other. Only problem is that's not an effective use of the amp, as the rears that fit in the factory location are very small. 20 watts is all you need there so I would suggest you wire the rears upto the Headunit amp rear channel output, then run your components off the amp with 4 channels bridged into 2 for max power to the front components. This would be a better setup as your front stage is your focus, so send the power there.

Third option is if your using a sub, then I would have the rears off the HU, the components off 2 channels on the NX4 and bridge the other two channels into one for a subwoofer.

This I think would be the best use of the power you have available in the car. Giving the power where it's needed most, highest power to the sub, medium power to the components and lowest power to the rears.

This setup would give you full fader control and subwoofer control also. Only other suggestion would be to look at a 2ohm component set to pull maximum power out of your amp. If your a tweeker then get a Digital EQ from Audio Control with external interface, otherwise don't bother your HU can tweek the basic treble and bass settings.

Just my suggestions, not a huge amount of options in a little car like the Z.