The spec of the iSTAR Fusion is 4 x 10MP cameras, and the spec sheet states that the panoramic image is 50MP, is this correct?

The iSTAR captures 4x 10 megapixel images.  When these images are corrected for lens distortion and the blank area at the nadir is included, the resultant spherical panoramic (2:1 ratio) image becomes 10,000 x 5,000 px = 50 megapixel.

A good resource for calculating an image size may be found here.