I am sure that someone will correct me if I am wrong (I don't keep as up on these things as I used to), but I think that google only show PR4 or higher links as back links.
This means that if your PR dropped from 5 to 4, the other links are likely to be at least one lower still and therefore not shown.
In short, sounds like the lack of internal links showing is a symptom of dropped PR and not the cause of it.
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