What does Bottled in Bond mean?

The law

The Bottled in Bond Act let distillers bottle whisky while it was still held in bond, before the tax was paid, as long as a government officer watched. The strip over the cap was the government's seal. Distillers used it as a sign of honest whisky, and the ads said so within a year.

What the green strip tells you

Before July 1937 the green strip was printed with the distillery's number and state, the distiller's name, and the year and season the whisky was made and bottled. Those seasons can date a bottle to within a few months. A law of July 9, 1937 cut what the strip had to carry down to the size and the words bottled in bond. Separate bonded strips ended on January 1, 1980.

You can watch the strip change year by year on the 3D bottle on the home page.

Does it mean four years old?

Not under the 1897 law, which set no age at all. A four year minimum was in place by 1937, from a later rule. The full list of rules, each with its federal source, is on Date your bottle.