The D4 is a processor only in the sense that it makes adjustments such as rectification to the raw camera data. It does not run an operating system such as Windows or Linux or have onboard storage for programs, and attachment of the camera to some kind of computing device with a processing chip is usually required.
The D4's purpose is to reduce processing load on the computing device's processor chip and avoid the need for a dedicated GPU graphics processing device by performing the depth computation.
Thank you for the reply I get that part, but doesn't it load some type of firmware from the spi flash? What if wanted to use my own camera board and space the cameras differently, or something like that? Wouldn't that require changes to the firmware on the D4? I was under the impression the D4 was also doing the disparity calculations to offload the processor?
According to the 400 Series data sheet document, the camera's firmware is stored in a 16 MB serial flash on the D4 (technically you could fit other small programs in that, so I stand corrected about storage)
However, I have seen no evidence that the D4 can be successfully paired with a depth module other than one in the 400 Series. There was once the suggestion that custom RGB sensors or tracking modules could be attached to the D4 via its second 50-pin connector, but Intel decided in March 2018 not to allow customizations on this connector, and the firmware does not support it.
The best option for a custom setup may be to choose a 400 Series camera module board that fits your needs (for example,the chart below shows that only the D415 module has an RGB sensor), as they are available in different configurations.
The D435 USB camera only has an RGB sensor because it is attached separately, whereas the D415 integrates the RGB sensor onto its board.
Since the firmware is stored on D4's flash, technically you could probably find a way to write your own firmware to it and talk to your own hardware connected to the 50-pin connectors, though it would be hard to get something working as Intel does not open-source the firmware and so it would be hard to know where to start in programming a custom firmware. And Intel would certainly be unable to provide any technical support for doing so.