A4 or A-4 may mean:

• A4, an international standard paper size (210×297 mm) defined by ISO 216.
• The A4 road in England that runs from London to Bristol.
• The A4 motorway in Switzerland that runs from Schaffhausen to central Switzerland.
• The A4 highway in France that runs from Paris to Strasbourg. It is a part of the European routes E50 and E25 .
• The LNER's Class A4 steam locomotives, including the famous Mallard.
• A4, the actual model number for the German V-2 rocket.
• Audi A4, a brand of car.
• The Douglas A-4 Skyhawk attack aircraft.
• A4, the IATA code for Southern Winds .
• HMS A4, an A-class submarine of the Royal Navy.
• USS A-4, a Plunger-class submarine of the United States Navy.