What do you think of this argument for God?

[Mister Agenda]

I heard an argument like this:

1. God is the greatest possible being.

2. God is a necessary being, which means that God exists in every possible world (If God exists). 

3. If God exists in one possible world, God must logically exist in every possible world. 

4. Since God is the greatest possible being, it follows that every aspect of God (being possible) exists in some possible world.

5. Therefore, God exists (in all possible worlds, including ours).

I actually just structured the premises this way myself but is the same idea as an argument I heard before. 

What do you think of it?

1. That's an odd definition. The common definition is along the lines of 'the creator and ruler of the universe'. This definition just calls whatever the greatest possible being is, 'God'. Maybe the 'greatest possible being' is a supercomputer made out of a network of neutron stars connected by wormholes. According to this definition, that supercomputer is God. I don't think that many people who believe in God would agree that the supercomputer is God.
2. It's entirely possible that there is a necessary thing, that 'can't not exist'. There's no reason to think that thing, if it exists, is a being. Quantum foam is a candidate for the position.
3. If God does not exist in one possible world, God must logically not exist in any possible world. There seems to be a missing step here where demonstrating that there is a possible world where God exists would be helpful.
4. How so?
5. There doesn't seem to be an actual argument preceding this conclusion.