The show runners have stated several times that they had an end game in mind from the start and the concept was always 5 seasons. They'd rather go out on top, on their own terms.
"This organism and derivative genetic material is restricted intellectual property."
Seems like it was a mutual decision between the creators and the network(s). The creators have said that they planned for five seasons but at other times at least Graeme Manson said that they could keep going as long as there was a story to tell.
The ratings for the last few seasons were lousy and I was especially concerned when the ratings took a big dip in season 4 despite moving from a terrible time slot on Saturday night to a prime time slot on Thursday night. It seems like an expensive show to produce(despite only having to give Tatiana one pay check)so I think cancellation was a strong possibility. I'm glad they get to go into season 5 knowing it's the end and not have to hedge or hold anything back in case of renewal. But I'll miss the show and the characters like crazy and I'll cling to hopes of an Alison/Donnie spinoff.
I'm glad they get to go into season 5 knowing it's the end and not have to hedge or hold anything back in case of renewal.