If you want to have an organised tour or rent a 4x4 yourselves you can contact Off-Road-Greece. Their page is in Greek, but scroll down to the "Book a car" button and the form is in English, which you should fill in and they will contact you. Some of their site is in English and it will give you an idea about what is in store for you!
An alternative to the above is Off Road To Greece.
For 4x4 enthusiasts based around the Vermio mountain area you can see more and contact Nomad Offroad.