Yes, on locations except on top or to the left (or on top and a bit to the left) you should use four gas miners, if you can spare the extra worker (which can be one reason for Zerg not do it) and your build order/strategy requires maxed out gas income (many Protoss do not even take all of their geysers in late game PvT, so building another Assimilator would be more efficient than using an extra worker on an existing one).