Audio Setup

Quick car audio question in general. I'm kindof a noob here still, I've been reading a lot but it gets complicated easily.

I have 4x100W RMS 4Ohms speakers. And my HU is obviously not powerful enough (4x40w RMS). It has 2 preamp outputs so I could wire up a 4x100w@4ohms amp (4 channels). The question is:
If I want to use a 2 channel amp, because cheaper and smaller, would I have to setup a 2x200W@2ohms and parallel wire both left and right speakers?
If i understant correctly, parallel wiring reduces impedance, so the amp has to cope with lower impedance output, 2Ohms in this case, since the speakers are 4Ohms x 2. And since they are 100wRMS, the output should be 200w RMS, right?

just amp your front speakers and have the rears running off the head unit.

music sounds better when its coming from in front of you anyway.

Can I bridge the speaker outputs for the rear? The impedance should stay the same and the power double right? I'd have 2x80W RMS in theory.

Your headunit won’t be 40rms more like 15rms if your lucky and you can’t bridge headunits it will just overheat

Just get rid of the rears and spend twice as much on decent front components and run a 2 Chanel amp

It's an FD high power, the 40w is precisely supposed to be the real rating, not the 100w. This is what it says on the manual:

Maximum power output:
100 W × 4 ch/4 O (for no subwoofer)
Continuous power output:
40 W × 4 (50 Hz to 15 000 Hz, 5 %
THD, 4 O load, both channels driven)
Load impedance: 4 O (4 O to 8 O allowable)
Preout maximum output level: 2.0 V

I don't really want to get rid of the rear, they give nice ambient sound. With only the front it seems flatter. But an amped front could give me the option to put a small woofer in the rear eventually, using the HU rear channels. So I'll probably go with that.