My oxygen dependent 5y.O with trach weighs 36lbs. A small m6 tank in backpack weighs 4lbs. Is it ok for him to carry on own to play 30+ min outside?

Boy with O2. Without knowing all the facts, but assuming your boy can carry the 4 pound m6 tank in a backpack inside the house with a problem, then it would seem to be a very good idea to let him play outdoors in a safe place. He should however always be supervised to keep him out of trouble.
Usually. If he hasn't played outside while carrying it before, i'd suggest starting with only a few minutes at a time, and building up gradually as his exercise tolerance improves. Check to make sure the outdoor air quality is good, and an adult should always supervise him, both to ensure he stays safe and to notice if he starts having respiratory difficulties.