Despite being a feature added in the latest 1.7 firmware update for the PlayStation 4, the ability to pre-load games won't start until Destiny is released in early September. Yes, that means that pre-loading will not be available for another four months from now.

This reveal came after Sony updated their Knowledge Center page on pre-ordering.
Pre-load pre-ordered content
Beginning with Destiny, certain pre-ordered titles can be automatically be pre-loaded prior to their release. This lets you rest easy knowing that your game will be ready for play the second its release date hits.

Please note:
  • Only titles designated as having pre-load functionality at the time of pre-order purchase can be pre-loaded.
  • Pre-loading does not grant early access to the game; it simply loads it several days prior to its release so you're able to play it on its release date.
  • Automatic downloads must be enabled for your PS4 to pre-load your game. Follow the instructions in the section above for instructions on how to make sure this feature is enabled.

It was mentioned previously that Watch Dogs would be able to be pre-loaded on the PS4. However, a DualShockers' chat with Sony support confirms that this is not going to happen.