OCn+OC7 is hardware PWM solution. Once you set up PWM duty cycle and period, timer generates PWM waveform without any help from CPU side and software. From CPU you only change duty cycle setting when required. You can have up to 7 PWM channels from 8channels timer. Of course, period of all these PWM channels will be the same.
Yes, you can generate PWM using just OC4, but at the cost of 2x interrupt events per PWM period. Provided TCRE bit is clear, you set up timer to set TC4 pin high on channel4 output compare at TC4=TC4+pwmlo and wait for TC4 interrupt. When interrupt happens, you change set up to set TC4 pin low on output compare at TC4=TC4+pwnhi; and so on, updating TC4 every time you receive TC4 interrupt. As you may guess, this narrows your duty cycle settings, so that they can't be too close to 0 or too close to PWM period. So from all available PWM settings between 0 and period, 0-N and period-N settins won't work, where N is interrupt latency time expressed in PWM pulse width/period units.
