If you can’t tell whether a scene is better before or after that 4th rewrite, stop. Like a doctor who takes the Hippocratic Oath, rewriters should “first do no harm”.
Screenwriter Rob Ramsey (“Life”, “Intolerable Cruelty”) offers this: “There’s some old canard about Jackson Pollack; somebody asked him the difference between his work and what a chicken might create with some paint and her feet, and Pollack said, ‘The chicken doesn’t know when to stop.’
Unfortunately, most of us are chickens. Thanks to the miraculous power of computers we are free to revise until, in many cases, we’ve lost all original meaning and vitality. I recommend referring back to the original outline to remind yourself of all those ideas you had when the project was fresh. Are your revisions continuing to improve on that which they replace? If so, keep up the good work.”
If not, don’t be a chicken; be brave and step away from the writing.
{I Haven’t been able to find the original source for this quote – if anyone knows, please post a comment or contact me. Thanks.)
GREG MILLER - Author & Writing Coach 
My current home is right next door to a chicken coop, so I literally hear chickens constantly to remind me. Thanks, Universe.