I want to wire 4 150watt lampposts what size off armoured cable do I need to use.What depth does the cable need to be buried.

"armoured" would indicate Canada or the continent. 120 or 240VAC? 600 watts isn't much. Burial depth depends on your electrical code.

You can't bury armored cable.

You can't bury armored cable.The OP is almost certainly in Canada. They have a commonly used cable, Teck (sp?) cable, that is a coated armored cable rated for direct burial. Here in the US we can get a coated MC cable that is pretty much the same, but no one uses it.