I've used these a few times, Hammers....................
http://www.ebay.co.uk/itm/8-X-75MM-FENCE-POST-SPIKE-HOLDER-75MM-METAL-HOLDERS-STAKES-GARDEN-FENCE-SPIKE-/221708619346
Drive one into each corner.
Run something hefty (maybe 6x2 or two 4x2s nailed together)from corner to corner along the 4m sides.
Set another metal spike in the centre of the 4m side to break the span.
Sit 4x2 joists across... supported on the 6x2s, or whatever you use.
Plenty of other ways of doing it, but it's a simple solution.