The claim here seems to be that the nature of an omnipotent being is such that it necessarily exists. This is what has been called an ontological argument, and the questioner has consciously tried to reproduce the logic of St. Anselm's famous argument for the existence of God.
I think that there are at least two points of vulnerability in the questioner's version. The first is that premiss (4) seems to be tautologous, rendering the enthymemic modus ponens a principio principii.
The second vulnerability is that the coherence of any claim of the existence of an omnipotent being is put into jeopardy by Plato's peritrope argument set forth in Theaetetus.
The questioner's premiss (4) might be restated as "If an all-powerful being exists, then he must exist in reality." The suppressed premiss is "An all-powerful being must exist." The conclusion is then "An all-powerful being must exist in reality." Since the conclusion is also a premiss, the argument looks like a principio prinipii.
Plato argues in Theaetetus, that certain universal propositions are false because they imply contradictions, which are false. His immediate target in Theaetetus is Protagoras' claim that all claims are true for those who make them. His argument runs thus: 1) If Protagoras' claim that all claims are true for those who make them is true, then the claim "Protagoras' claim is true" is true for those who make it. 2) If Protagoras' claim that all claims are true for those who make them is true, then the claim "Protagoras' claim is false" is true for those who make it. 3) Protagoras' claim that all claims are true for those who make them is both true and false, making it a contradiction. 4) Contradictory statements cannot be true. 5) Protagoras' claim that all claims are true for those who make them is not true.
We might put a claim about the existence of an omnipotent being into Plato's peritrope in some such way as this: 1) An omnipotent being can make an object of any mass he chooses 2) An omnipotent being can make an object so massive that it cannot be lifted. 3) An omnipotent being can lift any object. 4) Either an omnipotent being cannot make an object so massive that he cannot lift it, or he can make such an object. 5) If an omnipotent being can make an object so massive that he cannot lift it, then his powers of lifting are limited. 6) If an omnipotent being cannot make an object so massive that he cannot lift it, then his powers of object creation are limited. 7) An omnipotent being has limited powers or lifting or object creation. 8) An omnipotent being with limited powers is an absurdity and cannot exist.
So, the questioner's ontological argument seems to commit the fallacy of principio principii and imply a contradictory conclusion.