The Ontological Argument - valid or debunked?

[SteveII]

No. It does not work both ways because the meaning of P4' is very different from P4 because you cannot rely on modal logic of "necessary" as the original argument does.

So? Still valid argument, with conclusion following logically from the premises.

Even William Lane Craig acknowledges this:

Now you're absolutely correct, Randy, in noting that if we alter the first premiss to read
1′. It is possible that a maximally great being does not exist,
then the conclusion follows that
6′. Therefore, a maximally great being does not exist.
There's no fallacy here. The whole question is, which do you think is more plausibly true: (1) or (1′)?

Read more: http://www.reasonablefaith.org/necessary-existence-and-the-ontological-argument#ixzz4CoByxm58

WLC was asked if the argument was fallacious. It is not (otherwise that would have been my response). You still have to deal with the fact that the argument does not argue the both that God exists and God does not exist with the same logic because P4' is not equivalent to P4 as it relates to modal logic and "necessary".