2. Don't Sugar Coat it
A well-known fact is that Army of Darkness was going to end with Ash sleeping through his time period, and waking up in a post-apocalyptic nightmare. However, Raimi was forced to give the film a happier ending. For Evil Dead 4...don't. Army of Darkness is the lightest and softest of the series. Each of the previous films ended on a less than optimistic note. Evil Dead 4 needs to end the same way. No pulled punches. Leave Ash stranded in Hell if need be, so long as it's funny, with the future uncertain. There are two major reasons for this. The first being that Ash has to suffer. In all the films, he needs to suffer. That's where all of the horror and comedy comes from. An unhappy ending means more work for Ash, more for him to do, etc. This leads to the second reason: sequels. Army of Darkness tied everything up nicely, so part of the problem with a sequel is you need to ask "where do we go from here?" Do you throw Ash into an arena with Freddy or Jason? Does he get dragged back into business with a new Deadite outbreak? Does he fight Marvel Zombies? There's no clear cut answer. With a more ambiguous ending, that leaves the door open for more Evil Dead films, and that is never a bad thing. ...however, this is probably the most important aspect of them all...