Please don't spread misinformation. Retail games can't possibly utilize 4th core without CoreUnlocker:
1. Official Sony SDK doesn't expose CPU affinity mask value for 4th core for game developers.
2. Even if the devs would somehow guess the value, all thread-related functions have check in them that makes them error out if 4th core is attempted to use in game application.
The purpose of CoreUnlocker plugin is to disable that check.
It makes no sense for devs to use 4th core CPU affinity mask value (again, if they somehow guessed it), because all of the thread-related functions will simply error out. Games can't be programmed to use 4th core out of the box. That's the purpose of Thread Optimizer.
If some games do manage to run without CoreUnlocker and with 4th core enabled, that's because they have good exception handling and are able to work even if some thread-related functions don't work and return errors.
The reason you see 20 - 30% usage of 4th core without CoreUnlocker is because while games can't directly use it, SceShell and background applications use it, as well as some shared system modules.