If he plays the 4 offensively, probably coming off the bench, I could see him playing a lot like Pierce did with Brooklyn this past season. This entails popping off a lot of screens and taking threes and also driving off the catch. I suppose this will also be his game as a 3 as well but wing defenders will be more effective guarding him than bigs out on the perimeter. The concern is that he won't be able to hold up anywhere close to as well as Pierce did defensively because he doesn't have Pierce's long arms.
McDermott might be able to make the Dirk post fadeaway a few times but I doubt it's going to be an unstoppable move for him. Being able to make it against college defenders says nothing about his ability to make it against longer, bigger, more athletic and more defensively capable NBA defenders. Dirk is a 7-footer with long arms and a very high release; it make sense he can get that shot off and with his great touch, make it. McDermott doesn't enjoy the same physical gifts. Whether as a 3 or a 4, every defender is going to be as tall or taller and certainly longer than him. To think that someone with his physical profile is going to routinely score against NBA defenders in the post when post scoring is one of the most difficult things to do in the NBA seems unlikely.