In computers it is typical to define rules relative to data transfers for optimizing the overall system considerations.
One such consideration is to define coherency granules (CG) that relate to units of data that are stored in memory.
The magnitude of the coherency granule is typically significantly greater than the bus size.
In such a case, a data transfer of one coherency granule requires 4 cycles on the bus.
[1] As systems continue to get more and more complex, there has been a tendency to increase the size of coherency granules.