original docisis 3.0 cable modem can support up to 4x4, not 4x2. since the upsteam channels are limited to four channels that due lower band frequency that called sub channel band.
yes, a newer cable modems can do 8x4, again notice this 4 sub channels still there. it is very limited sub frequencies about 5Mhz to 50Mhz around.
I am looking forward the docsis 3.1 as i'm skipping the 8x4 modem.