Good question

it's certainly not made as clear as it could be!
I've double-checked with Hikvision, and the two sensors on the
DS-2CD2387G2P-LSU/SL and
DS-2CD2T87G2P-LSU/SL are each 4MP with a 4mm focal length.
The images are then stitched together to create an image that has a resolution of 5120 x 1440.