Now, from what I see, the ONLY possible way that they can have a Dead Space 4 is if they use it to finish the franchise. Which means Issac (and Carver) find the TRUE source of the Markers, find out how to disable it, or kill the moons, etc., and then end it. Forever.
Maybe it'll be better than 3?
Maybe there won't be many Necromorphs... maybe the creator race...
Maybe they'll both die.
And hopefully, it will be good
... And Earth gets saved... kinda...