I tried this here a while back, and didn't manage to convince anyone. But it's always worth a shot, right?
For what it's worth, I think that 4-6 are better parsed as follows (at least, for those who understand what philosophers mean when they talk about possible worlds).
4. God exists in some possible world. (Since it is possible God exists)
4a. If God exists, he exists necessarily (via the original Ontological Argument)
5. Therefore, in the possible world in which God exists, he exists necessarily. (4, 4a)
6. Therefore, God exists in all possible worlds (via S5)
7. Therefore, God exists in the actual world. (Since the actual world is a possible world)
The only obvious flaw is that it requires you to believe in S5. Now, I found S5 to be intuitively the best of all of the modal systems, but maybe that's just me.