Octave Number
The octave number increases by 1 upon an ascension from B to C, and "A4" refers to the first A above middle C, namely A440.
As an example, in ascending the white notes on a keyboard, C4 immediately follows B3.
Read more about this topic: Scientific Pitch Notation
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“States strong enough to do good are but few.
Their number would seem limited to three.”
—Robert Frost (1874–1963)