For my LSS, in 5W mode only the closest tube to the rect is used. In 15W it is the 2 closest and 30W all 4.
For you LS, i would guess that the 2 closest are used in 50W mode. (only guessing because of the similarities to the LSS, and the new LS has a 10W mode, which leads me to believe that it would be the same as the LSS??)