I think the answer would depend on the context in which the phrase in question is used. In other words, it would all depend on how you use them. As far as I can see, nothing is grammatically wrong with all four phrases, but of course, this should be expected because they're just that--phrases. A better comparison can be made if you were to use them in a sentence. I'm no English teacher but all four seem to be fine.
Difference of opinion: Only one dissimilarity over one opinion
Differences of opinions: More than one dissimilarity over more than one opinion
Differences of opinion: More than one dissimilarity on only one opinion
The first and third ones seem to be the ones that are used the most. But maybe that's because people focus on one opinion at a time when arguing. But it seems that they all are fine to use if used in the right context.
Thank you, for cleaing my doubt.
