The reason why Light Fields are considered 4D is because there is an extra 2D added for each position of the camera.
I'm a little confused about how the 4D light field works. With photographs, we can only capture irradiance on a plane (2D), but in the real light field, for each 2D set of sensors, it adds an extra 2D for each direction?
correct me if i'm wrong but i think the 2d vs 4d comes from the fact that "regular" 2d photographs are only taking the total irradiance at every pixel on the sensor (rightmost dotted line), so the only dimensions we consider are x and y. the rays still arrive from different angles/positions at the back of the lens, but that information is thrown away and we only need the irradiance at each pixel. on the other hand, for the light field we need that information--the position the ray comes from on the back of the lens, and subsequently the angle, so we need 2d for location on sensor and 2d for laction on back of lens
Photograph shows the result of lights, irradiance, while light field shows the movement of lights, radiance.