PS4 already has a low level api, this has been said in a few articles (I think the crew developers mentioned it for example), whether it's mantle is another matter, but PS4's situation is not that much different to the PS3 or Vita, both also had low level api's.
MSoft have said that DX has hardware specific extensions, but that's DX so I'm still unsure about them. But Mantle does support HLSL so it might be there? (edit: and would stand more to reason it's on X1)
edit2: ok not really sure how to write my reasoning for x1 xD, mantle could prob work on x1, especially as it supports hlsl. But I've not heard much about msoft's low level api stuff, just hardware specific extensions to DX. I don't know whether mantle is on any of the consoles, but at least the PS4 and MAYBE the X1 have SOME form of low level api for graphics. But I don't personally think either are mantle. Especially if BF4 is being patched with it in December? If either of the consoles had it (especially the X1) surely it'd be there at BF4's pc release!?