1 The normal one would be 'the American accent', but as there are many, 'a' could be used.
2 You could use both for the bus, though 'the 84' would be more common.
3 Most likely 'the'- a specific dress.
4 Most likely 'the waiter' as mentioning the restauran makes it specific, but if there are many waiters 'a' could be used.
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