From Wikipedia

An AAAA battery is 42.5 mm long and 8.3 mm in diameter, weighing around 6.5 grams. Output of alkaline batteries in this size is 1.5 volts, 625 mA·h.

This battery is also classified as LR8D425 (IEC) and 25A (ANSI/NEDA). The battery is also known by its Duracell type number MN2500 or MX2500 and Energizer type number E96.

This battery size is most often used in devices requiring small batteries such as laser pointers, LED penlights, powered computer styli, and small headphone amplifiers. These batteries are not as popular as AAA or AA type batteries, and can be quite hard to find. Some (not all) brands of alkaline 9 volt batteries have a bundle of six AAAA cells inside, connected together by welded tabs.[1][2] When extracted from a 9 volt battery, the cells have misleading polarization - the flat end is plus and the pointed end is minus.

The AAAA is also available in a PCB Tagged version, 8.4mm x 66mm, weighing 10.5g. These tagged AAAA batteries are NiMh rechargeable as such will charge from the PCB they are soldered onto.