I've installed about 3 different sound systems now, 1 in my car and 2 for friends, my experiences so far:
My first setup in my car had 2 amps, 1 x 2500w 4ch for the split system, front and rear, and 1x 4000w monoblock for the sub woofer. It was loud as in your ears feel like they are going to bleed (other cars windows would rattle when parked next to me at robots

) but it caused problems, the headlights dimmed very badly, the revs would drop on bass drops, and if the bass was really hectic the splits would be starved for power, most importantly the split systems were being drowned out somewhat.
My second setup was 1x 4ch amp for 2 split systems, no sub, wasn't impressive, but the split systems were on the cheap side.
Third setup I'm most impressed with, we got 2 split systems, Infiniti Cappa's and a single 2ch Amp (can't remember the power rating sorry). Then wired the split systems in parallel. It had no sub but it was hectically loud.
So basically my advice:
Firstly co-axle and 6x9's don't even begin to compare to a Split system, not even close. If you want loud + good audio quality a split system is the only choice. Btw. a Split system is basically a set you buy that comes with a filter that filters the mid range and high range, 2 tweeters and 2 mid range speakers, you wire them to the split.
Secondly, 4ch is a waste of time, let me explain why, a Amp may be rated 100W rms @ 4ohm and 200W rms @ 2ohm, so it puts out twice at much power at 2ohm. But most split systems run @ 4ohm, so if you wire them in parallel then you have 2ohm and get much more from that amp.
The down side? You won't have front and rear anymore, your two left speakers and two right speakers will be in parallel running from the HU's output so you'll have the same loudness front and rear. I never use the front/rear volume thing anyway so to me that isn't a problem.
Lastly 2ch amps are usually more compact anyway so it's easier to mount.
My 2c.