Based on camparing the venue map to the photos...
The available seating (front tier plus the second tier on one side of the mixing desk) is a bit more than 2,000. People present have reported it as 'almost full', so let's call it 2,000.
The official standing capacity is 4,321 (cool number). However, as has been reported, the stage was brought forward quite a lot to restrict the standing area. I'd say it looks like about half of what is available. So let's call it 2,160.5.
So that's 4,160 and a half. But this assumes no major gaps in the seating and a standing area that was packed, so that you would need to squeeze through people to move around. People who were there will know, and can make mental deductions accordingly.