Mrtvica Canyon is a lovely Canyon, which gives you stunning views and cold water on nice hiking oppurtunities.
I reccomend the hike from the village Medjurecje. There are different parking possibilities in the village and it takes 1,5 till 2 hours to the cache.
There is also a way from Velje Duboko, but i didnt have a look at it.
Due to bad mobile Connection i give you an additional explaining to the cache. There are also pictures for the last 50meters. You should have a look or safe them before/when you start the hike in the village.
After you have passed the gallery, take around 45 more steps until you see a big rock on the left. The cache is under a wooden plate covered with stones. Look at the spoiler, maybe you watch it already from the path. T4 is only becauso of the hike, there is no need to climb anything.
Have fun and say Hello to the unicorn in the logbook.
PS: In case there is nothing to find, please send me a message or make a note instead of making a maintenance call. That helps better finding a solution.