For most listeners, the two examples will communicate the same thing, that speaker B doesn't know the name of meal #4. Technically, of course, B might know the name but not know what the name is or what the meal is called, etc. Here is a classic example, that illustrates several distinctions:
"Or else what?" said Alice, for the Knight had made a sudden pause.
"Or else it doesn't, you know. The name of the song is called 'Haddocks' Eyes.'"
"Oh, that's the name of the song, is it?" Alice said, trying to feel interested.
"No, you don't understand," the Knight said, looking a little vexed. "That's what the name is called. The name really is 'The Aged, Aged Man.'"
"Then I ought to have said 'That's what the song is called'?" Alice corrected herself.
"No you oughtn't: that's another thing. The song is called 'Ways and Means' but that's only what it's called, you know!"
"Well, what is the song then?" said Alice, who was by this time completely bewildered.
"I was coming to that," the Knight said. "The song really is 'A-sitting On a Gate': and the
tune's my own invention." ...
Best Answer
There is no semantic difference between these two:
The difference is one of register alone, where the first is standard English and the second is perfectly common but far more casual, and is not generally considered acceptable in formal writing save as reported speech.
Please note that the second one is not a double negative, for if it were, it would be a positive! And it’s not. Consider this contrasting pair to see the difference:
I don’t know nothin’ about that. (reinforced negative)
I don’t not know anything about that. (negated negative)
The two components of this second pair are no longer equivalent. The second is-at last a true double-negative. The first is merely a single negative reinforced through reduplication, which is why it still has negative sense. The true double-negative alone has positive sense.
In English, a double-negative makes a positive, just as a double-positive makes a negative.