So far, I have always considered じゃないか to be a tag question meaning "...isn't it?" with varying nuances depending on context. However, I have seen hortative + じゃないか recently, in this short dialog (A and B are dining together):
(メドック is Médoc, a kind of French wine)
I suspect that this じゃないか turns the sentence into some sort of negative hortative ("won't we drink?") kind of like saying いきませんか？ However, I'm not sure how exactly じゃないか modifies this sentence. Does strengthen B's encouragement? Does it have a sense of "urgency" ("we really should have one more Médoc) or does it the opposite, "softening" the sentence?