Will this dog make a good guard dog?

I have a 4-month old puppy that i adopted at 9 weeks. He was a mixed breed, but the shelter said that they knew for sure that his mother was a German shepherd, but didnt know what the dad was. They said that it could have been a mastiff, since he came in with a brother who was Brindle. (he is fawn with a black muzzle). However, when i put his picture on Y! Answers, most people said that they thoght he was a great dane. I looked up pictures of fawn great dane puppies on google, and real;ized that he looks almost exactly like that. He is already showing signs of being protective, like if he hears anything outside, he will run to the front gate and bark his head off at it. Also, if my dad pretends to hit me, he will start growling and puts his head under my dads hand so he gets hit instead of me. This dog will NOT be used as a "guard" dog, but i just want him to protect us if something bad ever happens, like a burglar or anything, since i am home alone from the time i get home from school