well, let's take a looksie at the picture
ahemm, the blue lines or diagonals, have sides of either 6+6 or 4+4, they make up four triangles each one with a side of 4, a side of 6 and another side that's pretty much the same on all triangles.
what the heck all that means?
well, it means, the sides AB=BC=CD=DA, and it also means the diagonals are bisecting each other, namely cutting each other in two equal halves, wait a second!!!!! equal sides, bisecting diagonals!! that's a RHOMBUS baby! and yes, a rhombus is a parallelogram.