Energizer - AAAA Alkaline Battery for Laser Pointers, Penlights, Computer Stylus, and Others
Brand: Hitech

The AAAA battery is 42.5 mm long and 8.3 mm in diameter, weighing around 6.5 g. This battery size is most often used in devices requiring small batteries, such as laser pointers, LED penlights, powered computer styli, glucose meters, and small headphone amplifiers. The avarage AAAA alkaline cell has approximately a 595 mAH capacity.
(description from Wikipedia)

High quality Energizer AAAA battery
For small devices including laser pointers, LED penlights, styli (styluses), glucose meters
1.5 volt alkaline cells
Approximately 400-500 mAh
Equivalent to LR8D425, 25A, MN2500, MX2500, E96