During Upper Mustang Trekking, trekkers hail upward to the height of 4000 meters (and somewhere to 4500 meters). The trekking up to 5000 meters is marked as a moderate level of trekking. The altitude sickness such as AMS (Acute Mountain Sickness) is probably not common in such height. But, prevention can be always done, with carrying AMS cure in first aid kit.