We have obtained (in BVRI filters) the structural parameters of four disk galaxies, spanning a range of different morphological types. We believe the parameters are reliable because an extensive part of the galaxies' disks are free from contamination by other components, and a detailed photometric method has been used to derive these quantities. Several correlations between their structural parameters have been found which support: a disk increasingly optically thick towards the center of each galaxy; the older population of the bulge being more extensive than the younger; and a `Scaled' Hubble sequence for the spirals.