The 'recipe' I've always used for a bedding mix of mortar is 4 parts sharp sand, 2 parts soft builders sand and 1 part cement with enough water to create a mix that is workable, but not so sloppy that slabs laid onto it aren't supported by it when its wet. If the area's quite big, a bit of plasticizer added to the mix keeps it workable longer.

I agree with Bamboo, but have 1 question, how on earth do you lift a 4 metre square flagstone?, I assume you mean 400mm lol, Derek.