Sorry, I don't know the answer to that. I was talking to Mark Alpin, sales manager for toy haulers, and that is where I got the sf figure. I mentioned that the newer RW and Cyclones where longer than my 40'5". He said to ignore the length, the sf was the limiting factor in their designs. He didn't say if that sf was with the slides extended or not. However, if I calculate the sf of my RW with the slides closed it comes to just under 345sf. So, I would guesstimate that the total sf is with the slides open so that would make the figure of 619sf for the 4200 wrong. Lou