Rather than guessing why not read what the guy who took the photo has said as he was there and did not have to guess.
Well just because he was there to take a picture doesn't mean his information is accurate. I spent some time as an aircraft refueller and while I was only present for servicing of a 747 once, I have never come across a crossfeed valve at the fuel station on any aircraft.
I have however had occasions where a tank needs to be drained (into the center tank or other wing) for maintenance purposes, for example to allow for a known quantity of fuel to be added to the empty tank.
IN this picture the steps next to the number 4 engine suggest to me that there is possibly some work being done which required an empty tank, quite possibly fixing a leaking tank access panel.