The fan on any Source Four LED fixture can be controlled via DMX if needed. Does this mean the fan could always be off for example ?
No. Assuming the fixture's DMX mode is set to allow fan control via DMX (see link to manual below), the fan can be controlled in a range from 0 (off) to full speed.
However, the fan will be turned on to protect the fixture if a certain temperature limit is reached - even if the DMX value for the fan is 0. In normal operation, this is not expected to occur very often.
Quote from the manual:
The described behavior is for firmware version 1.8.0 .