1234: just avoid  running too much on hard surfaces, if he is living in a wide fenced area there is no need for exercising. at 4 months if you want you an walk him for 20 minutes or play fetch for 10minutes you can do that 2-3 times a day. later at 6 months you can walk him longer. Volka is 9 months i take him for 45 minutes walk which includes some playing and running, then later during the day i take him for short 20 minutes walks....but because he lives in a house. soon i am moving to my summer house he will have a fenced area. when he gets older around 2 years you can then start some serious exercising with weight pulling etc....

but as said in this thread, if it is hot the dog will be sleeping all day..they are lazy by nature. and start obedience early and do not encourage play biting , this is a serious dog you dont want him to play biting as he gets older trust me... they grow very fast , so you have to control them early.... Volka is now 9 months and 135 lbs 30+ inches at withers