You can drive to laguna Miscanti from San Pedro in few hours on good paved road and then on good trail. You don't need a 4WD unless it snows. A your arrival at the rangers' hut at the laguna, you have to register and to pay an entrance fee (quite cheap). You'll find a very comfortable and clean refuge if you want to stay under a roof (with warm water !!). Miscanti is considered as a sacred mountain and is therefore not allowed to be climbed. Miscanti is a nice summit seen from far but nothing more than a big pile of rock ! When I was there, I took the wrong way (left of the mountains) and had to climb 8 hours in steep and unstable plate like shaped rocks. The right way is : when you leave the laguna, go a little bit on the right of the mountain, even if it seems a bit steep. The higher summit it the one on the back. If you need some detailed photographs, you can contact me.