Are 2 2x4s stronger than a 4x4?

When used vertically, 4x4s are stronger than two 2x4s. However, if you need a horizontal surface, two 2x4s will be stronger than one 4x4. A 4x4 should not be used horizontally for anything structural. Always be sure that you're using the proper size and strength of lumber. Using two 2x4s is not the same as a 4x4.