What makes you think that? Unity uses: D3D9, D3D11 (new in 4.0) or OpenGL on Windows; OpenGL on Mac & Linux; OpenGL ES 1.1 or 2.0 on iOS, Android and NativeClient; libGCM on PS3; D3D-like API on Xbox360; Stage3D on Flash.
What is new in 4.0 is D3D11 support, which is kind of a big deal since it opens new GPU possiblities (compute shaders, tessellation etc.). But all the other rendering APIs are still there and fully supported.
What is new in 4.0 is D3D11 support, which is kind of a big deal since it opens new GPU possiblities (compute shaders, tessellation etc.). But all the other rendering APIs are still there and fully supported.