A D4-brane is a D-brane with four spatial directions and a five-dimensional worldvolume, belonging to type IIA superstring theory. It supports the low-energy modes of open strings ending on it, while also coupling to bulk closed-string fields.
Its electric coupling uses a five-form Ramond–Ramond potential, dual to the three-form potential coupled electrically to a D2-brane. Thus brane dimension determines which differential-form field measures its charge, not merely the size of a geometric object. At long wavelengths, an isolated brane's slowly varying fields are described by a Dirac-Born-Infeld action, with transverse scalar fields encoding its position and fluctuations.
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