Although Remote Play from the PlayStation Vita to the PS4 has been confirmed, Sony opened up a bit more today at E3. To clarify the situation, Sony has informed us that Remote Play is basically required for PS4 development. This is due to the fact that it's taken care of system-side, and the single biggest issue on why there aren't many PS3 games that utilize it is because it was optional. In order to take advantage of the feature, the game has to set aside some memory or CPU related cost -- and memory is the most precious resource.
When it comes to the PS4, features are very easy to drop -- "it just happens" Sony says. Sony is asking to make sure every game plays well on the Vita, and they provide default mapping system-side, but it's always better for the developer themselves to make the decision.