If the 2X are laid down on the 4" side, yes, support will be needed. But if one were to turn the 2X4 on the narrow edge, it is much much stronger. So two ways I might suggest. One is a rectangle with the 2X4 on the narrow edge and held from tipping over by adding a 2X4 at the ends. A block support will certainly make it strong enough but that takes space. I would go with 2X4 laid flat and then support the middle with upright 2X4 "props". Wood standing on end is extremely strong if it can't bow, so even something as thin as 1/2" plywood as a center support will do the job IF it is made stronger by adding a plywood or 1X 2 at the front and back of the plywood. Think of it as a wooden upright I-beam?
Which works best for you will depend on how you feel about an easy stack of blocks but lost space or a bit more effort to do props but waste less space.