To enable 4 cores:
sudo nvpmodel -m 0
No reboot needed
To go back to 2 cores:
sudo nvpmodel -m 1
Power modes are listed in /etc/nvpmodel.conf
My understanding is that power mode 0 (4 cores) can draw more that 5W, which is as much as the board can get from the power source connected to the USB port, which is what is likely to be used when running the board from batteries.
When running the Nano from batteries, the CPU power must therefore be reduced by more than half (power mode 1 also limits the max clock).