Thanks, great ideaYou can use 1 part mica to one part oil on newly made soap. The oil gets reabsorbed. Not sure it will work on 4 month old soap.
You can make another batch and use a tiny amount of the batter as a paint or with a piping bag to add decoration like a cake. It sticks well to new soap - not sure about 4 month old soap but you have nothing to lose trying it.