You can use a KGT Drop Door Kit to place your Nelke PRO 4 Greenhouse on a 20" knee/stem wall. The kit has aluminum pieces to replace the vertical extrusions on either side of the door with taller extrusions that run from the ground all the way up to the eaves in one continuous piece for added strength.  The kit includes a piece of polycarbonate to fill the header space above the doorway.