My suggestion is when creating a new level on PS4, the player can have an option to make either a PS3-friendly but more limited level,  or a PS4 exclusive game with touch-pad control support as well as controller position and angle sensing and light color manipulation support. The PS3 friendly mode would be accessible to PS3 users and use PS3 features.

However, opening a PS4 level allows the utilization of new features and tools that utilize PS4 features, like touchpads, controller lights, and a larger thermometer because the PS4 can handle more, as well as higher sticker and decoration limits, along with support for getting and reading controller movement and angles. Maybe it could also utilize mouse and keyboard support in some way, kind of like how Move controller support worked, with cursors and keyboard controllinators added respectively to open new control possibilities for creators.

If you haven't already seen it, I also made a post entitled New Logic Ideas. Please read it and support! Sumo, please consider these ideas.

The game itself could run even on PSX.
There is no reason why levels from PS4 should not work on PS3.
You dont believe me?
Then play high performance games on PS3.
Play Uncharted3, Killzone3, Crysis3, Tombraider or whichever egoshooter game on PS3 and tell me how can LBP3 need more performance.
There should be enough high end games to compare that are beautifully optimized and work without bugs.

I mean, you know how right now the PS4 controller has all these cool features, but we can't do things like use the touchpad or use it like it's a Move controller in our games?

The idea is to add support for the PS4 hardware at the cost of no PS3 players being able to access this specific level, simply because their controllers don't have these features.

Any level that you pick not to use these features in would still be accessible to PS3 users as normal.