Portable is pretty easily defined as "carryable", from the literal French. In the context of software, it means between operating systems and hardware. Writing portable code is generally a good idea nowadays.
The engine (and its features) defines the format of the assets. Tessellation being one example. On that note, the polycount "during gameplay" is scalable already on PS3, too. That doesn't mean it couldn't be higher on average on PS4, because it will be.
PD use a lot of shaders for materials, and they've been doing it since PS2 in a limited form. PD have also been using high-dynamic-range texture spaces on all the PS3 games (it was faked on the PS2 games for the night tracks), and their lighting is nothing if not physically informed. I'm sure PD know to use photographic colour and metering references alongside their photographs of cars and tracks.
To sum it up: there are a lot of features in the way GT6's technical underpinnings work that point to work already done for PS4, certainly in a schematic sense (asset format), if not the nuts and bolts (lines of code).