I'm not sure how it works, but I think everyone has to stay busy. So, when they have finished what they are doing in a GP or EP etc. they move on over to a different pack. I think they already know what is coming next long before they ever finish a GP or EP. Or you would have some departments twiddling their thumbs waiting on work to do. I don't think they can manage this game that way. I think every department of TS4 has to stay busy with no breaks in between or you would be paying people for doing nothing. So, it stands to reason @Dannydanbo is more right than wrong. :)
Example: If the FX guys have finsihed what they were supposed to do for a GP or EP, then they would be just sitting there while others continued to work on the pack..so they have to know what is next and move on as others finish their own work in a GP or EP.