“4U” refers to the height of a module or rack format, with “U” being short for “rack unit.” A rack unit is a standard unit of measure for the height of equipment that is designed to be mounted in a 19-inch wide rack or a 23-inch wide rack. One rack unit is equal to 1.75 inches (44.45 mm) in height.
The 4U format means the module or rack is 7 inches (177.8 mm) tall, as it is four times the height of a single rack unit. The 4U size is commonly associated with the Buchla and Serge modular synthesizer formats, which are sometimes referred to as “Buchla format” or “Serge format.” These formats have a slightly different design aesthetic and functionality compared to the more common Eurorack format, which is 3U in height.
Modules in a 4U format often feature banana connectors instead of the 3.5mm connectors typically found in Eurorack modules. They also tend to have different power supply requirements and may include additional features, such as CV/gate processing or audio processing, that are not found in Eurorack modules.