Why are there 4 hands on the throttles of this B17 during takeoff?
If you look at the throttle quadrant without all the hands involved, you can see more detail:


Source

At the bottom, the 4 levers control the pitch of the propeller blades. The FE is holding them to ensure that they are at full pitch for the entire take-off run. 

The large levers in the middle are the throttles which (partly) control the manifold pressure (and power) that the engines are producing. Two pilots have hands on the throttles to ensure that all 4 engines are producing full take-off power.

But there are 4 more levers that nobody has their hands on, which control the waste gates for the turbocharged engines. These open or close the waste gates, and allow the turbo to make power. There are another 4 levers on older consoles that control the mixture settings. The levers shown above may be the mixture levers, its difficult to tell from this picture because it looks like the quadrant has been upgraded.

If you want to see a good video on how all these levers work, here is a really good YouTube video describing how to fly a B-17, start from about the 2:30 second mark, but its an interesting watch.