Ok...per the Captain:
The B747-400 taxis on all four on departure taxi, especially when heavy(I.E. Trans-pacific Departure). They have a N1 limit of 40% for taxi.
They can taxi light on three engines. Again, they have a N1 limit of 40% on taxi, so sometimes, a three engine taxi will not work.
They usually push back before starting ANY engines.
The only time an engine is started in the gate area, is when they have an INOP APU. And in that case, it is number four that is started. Number four has an axillary hydraulic pump that feeds brakes and steering.