  This paper is the fourth in a series, studying the optical properties of a
sample of mid-IR Warm Seyfert galaxies and of a control sample of mid-IR cold
galaxies. The present paper is devoted to the analysis of the colour
distributions characterizing the host galaxies. The Warm Seyfert 1 and 2
galaxies show opposite colour gradients and their colour profiles are depicting
age and dust effects within single-burst, solar metallicity models. In
particular, we find ample evidence for the occurrence of strong star formation
in the Seyfert 2 disks: their colour and emission line 2-dimensional maps
suggest dust extinction associated with on-going star formation in spiral and
tidal features; their colour profiles show starbursts of 0.5-1 Gyr or younger,
superposed on the older underlying galaxy population. Most of these properties
are shared with the Cold galaxies, while the Warm Seyfert 1s show mostly older
stellar populations and, in only a few cases, evidence for circumnuclear star
formation.
