HAES 4 channel vs 6 channel

[jmcl]

Hi,

before considering changing a 4 channel HAES for a 6 channel HAES can anyone please explain what is the gain on doing this mod.

My Box has the stock HAES 4 channel. I already changed the front dash speakers with better ones. That helped.
Door speakers are very low and with almost no sound.
I installed a double din unit but kept the factory harness intact.
Now considering rear speakers... Do I need the 6 channel HAES for this?
Txs.