The treasure is up the mountain by the farm. The treasure map indicates the location of the farm then gives you a perspective drawing of where up the mountain it is if you use the farm building as a reference. When you get up there it's just a chest behind a tree or something.
I created a topic kinda on the same thing (but when I searched I used "map IV"). I'm wondering if you are having the same problem I am? When I went to the right spot where the treasure should be it wasn't there!
If you look on youtube there is a coupe videos that show where it's located. I'm thinking there may be a bug in the system?
I found it, its just a chest hidden in some bushes. I didnt get anything special from it, just about over 100 gold, a few potions and some armour (no enchantments). so even if you dont find it i doubt your really missing anything decent.