Hello Stan,
The CCL ND 4 combo is capable of pushing the speaker to the 250W all in itself. On the other hand the CCL ND 8, has an 8ohms speakers, therefore proving more resistence for the amp, this means that you lose around 30% of the power from the output of the amp to the speaker itself. This is the reason why the CCL ND 8 has an external speaker output, in case you decide you want to plugin another 8ohms speaker (in parallel) and reduce the impedance to 4ohms.