Yes I am sure of it. If they are on a 20/4 light cycle then you are just taking 4 hours of light away that the plants can be using for photosynthesis. Before writing the first post though I double checked all the Ed Roshethal books that I own just to be 100% sure. There have been other people asking the same question, and he answers it the same all the time. He says studies have show plants under 24 hours of light grow faster and better, the 20 hour cycle offers nothing to prove it is better than full light.