The Fourth Painting

The Fourth Painting was an altered representation of one of Aguilar de Nerha's genetic memories.

Aguilar's Journal
We recovered the third piece from Lieutnant Cavallero's villa and Da Vinci's set is almost complete!

''The fourth piece was given to an Inquisition treasurer, Cristoval de Merlo, who is safekeeping it deep underground below the lums of Madrid. We must be stealthy and will need to disarm traps in order to recover it.''

Reference

 * Assassin's Creed: Rebellion