#define HD44780_4BIT_INTERFACE     (1<<1)

Setting HD44780_4BIT_INTERFACE as a capability flag returned by the capabilities method indicates that the data bus is interfaced with four bits (DB4 through DB7) and that the high-level driver should use four-bit mode. If this flag is clear, the high-level driver runs the physical interface in 8-bit mode which is faster than 4-bit mode.