A:Answer Cycle, options, and software aside, one can find that the WM3900 weighs 202.8 pounds while the WM4000 weighs 187.4 pounds. I suspect some metal parts in the 3900 have been replaced with plastic in the 4000. This is not necessarily a bad thing - it just depends on exactly which parts were changed or deleted from the design. I did call LG and specifically ask if some metal parts had been changed for plastic, but I was not able to find out what specific parts had been changed.
