Do you think the 4th egg is viable? 
Not sure - it is very messy, could just be that it got pooped on and feathers & debris stuck or it could be that the egg cracked and feathers & debris stuck to fluids that leaked.
Time will tell - or the peregrines will roll it out from under them. If I had to guess, I think that if it doesn't hatch in the next couple of days, it likely won't. Wouldn't have thought that long except that Mistral took a week to hatch ...
