The latest instalment of RTE's gangland hit finished on Sunday night and saw an average of 1,007,500 viewers tune in, 54,500 more than last year's final episode.
Despite the high numbers, fans took to social networking sites Twitter and Facebook to express their frustration with the closing episode.
Although audiences saw the demise of bankrupt dentist Andrew (Peter O'Meara) and teenager Wayne (Barry Keoghan), seasons two and three rounded out with the climactic gunning down of leading characters, such as John Boy (Aidan Gillen) and Darren (Robert Sheehan).
Mr Caffrey told the Irish Independent this was the first year the team went into production knowing another season would be commissioned by RTE.
As such, he said creator Stuart Carolan didn't feel the need to "book-end the finale with a dramatic moment".
"The mindset was that we'd make something to run over 12 episodes, as opposed to six," Mr Caffrey told this newspaper.
"We're expanding the storylines over a longer period of time and we didn't want to do a 'shoot them and gun them all down' thing again this year because we've everything signed up and ready to go for season five."
He continued: "Apart from Debbie, none of our long-running characters saw their demise, but we couldn't keep doing the same thing over and over again and playing that same tune."
Meanwhile, Fianna Fail TD Willie O'Dea has labelled references made to Limerick in the finale as "grossly unfair and unwarranted".
The former minister has called on RTE to clarify comments in which the city was referred to as a "kip".
Mr O'Dea said: "This was grossly unfair and wrong. I know people in Limerick feel very aggrieved by it."
He said the comments were even more damaging because they were made by gangster Nidge.
"These were lazy throwaway lines thrown out by people outside Limerick to entertain the public. All the major Limerick gangland figures are locked up which is more than I can say for gang leaders in Dublin."