We may exist in more than one dimension, but that does not mean we comprehend the others naturally. If we did, then we would be able to put a name to them - or at least describe them.
If we saw things in four dimensions, we would be able to look at how shiny a rusty sword was 400 years ago just as easily as we might turn it around to seen if the other side was any different.
The omnipotence comes not from existing physically in more than 4-dimensions, but existing mentally in more than x+4 dimensions. We don't know just how many there are, and being simultaneously conscious of all of them, seeing things as they are projected in each one, would have that omnipotence. Maybe what the omnipotent being might be works in a completely different way than we do; maybe it works largely by using geometric laws exclusive to its highest plane, and projects in our highest dimensions as something else entirely. Something that we may mistake for something else, while still being what we see. We could in fact be component particles of this consciousness, our actions independent of it, affecting what it thinks and does, but it not being restricted by us because there are things other than us that could also compose it. But we just don't know.
If I'm not being clear, sorry, that's the best way I can think to word it. What I'm basically trying to say is that things might look very different if we saw more than 3 dimensions. But right now you're getting dangerously close to trying to prove religion wrong, which is not the point of this thread. The point is to discuss various explanations of religion or the lack thereof. Not argue about them, which was going along just fine.
Remember that the view I try to take is the one in the middle, as I have said before. If you try to argue about it then you are trying to eliminate the possibilities that are not yours, unless you make a suggestion to improve or add to it.
Edit: There are things a 3D object can do that a 2D can't. Also, somethings we see as 3D could potentially be made to look like a 2D, through the use of lighting angles and such, and hide the 3D aspects. I have seen this done with a building before; I could have sworn, looking right at it, that it looked just like it was made of cardboard. Moving enough to the side or towards it, it then looked 3D again. God could just be pulling that sort of trick with more dimensions, and we lack the tech or mental capacity to "move closer," so to speak, as we do not know how to change our position on those dimensions at will. So he might not be and might not exist. We just don't know.