One core for the home menu, one for the face tracking on the super stable 3d, 70% of one for userland functions implemented in system APIs (such as sound and button input), and 1.3 cores for games.It couldn't use all 4 cores for gaming. If that were the case then pausing out to the home menu would be impossible since one of the cores is used to run that in the background. I'm fairly sure another core is being used for security measures seperately from this as well.
The O3DS has the last two, and includes home menu processing in that 70%.