filbert

You will need at least one angle to do the calculations, or you can measure one of the diagonals.  For instance, do you have at least one right angle in the plot?  If so you should be able to use Heron's formula for the areas of the two triangles and then add them up.  Also, you can use Brotschneider's formula, but I believe you will still need at least one known angle which could be derived from the length of a diagonal (connecting two opposing points crisscrossing the quadrilateral).

Then it would simply be a matter of plugging your square meters into square feet conversion engines that are plentiful on the web.