No matter how many tours a soldier does, it does NOT guarantee that they will get PTSD.
I must agree with this, and with what becvan said as well. I am not an expert on the subject except that my girlfriend comes from a strong military family, and I reckon her family is a good example to use here. Her father, uncle, and grandfather are all veterans. Her father served the longest, 40 years, and is recently retired. He was on countless deployments, even being in Afghanistan shortly before his retirement. He has been in combat, injured by bomb blasts, shot, a POW, the full experience. Yet he does not have PTSD. He is one of the happiest, sanest men I've met in my life, in spite of serving 40 years active duty. However, his own father suffered PTSD from the Korean War, after only serving 3 years. His brother also suffered PTSD and ultimately committed suicide, yet his deployments and years in the service were limited by comparison.
I believe in my girlfriend's family, there is likely something genetic at play, as more than one person in the family suffers PTSD, including herself, though her PTSD is not combat related. However considering that her father was the longest serving member and saw the most combat, yet does NOT have PTSD, makes your theory seem flawed in my humble opinion.