The Trails of Cold Steel series, itself a part of the long-running The Legend of Heroes series, is a bit of an oddity. While most of The Legend of Heroes series are trilogies, Trails of Cold Steel is going to get a fourth entry, which will completely wrap up what Falcom called the Empire Arc. A recent interview with Falcom President Toshihiro Kondo gives more details about the decision for the fourth game. The interview was originally posted on Ryokutya2089, but Gematsu has posted a summary of the important bits.
Gematsu directly quotes Kondo involving Trails of Cold Steel IV:
?We actually intended to end the series with The Legend of Heroes: Trails of Cold Steel III, but we thought it would be tough to end with one title and made a difficult decision.? ?(The next game) will truly be the last. We?re working for all the mysteries of the Empire Arc to reach the end of the line.?
At least we know that Trails of Cold Steel V won’t be happening!
Trails of Cold Steel IV was announced by Falcom themselves recently, as well as announcing that they would be bringing all of the Cold Steel titles to PlayStation 4. Here’s more on that:

The newest title of The Legend of Heroes: Trails of Cold Steel series [III], which has received numerous awards including Japan Game Awards, PlayStation Awards, and Famitsu Awards, is planned to be released on the home game console PlayStation 4.
Regarding The Legend of Heroes: Trails of Cold Steel and The Legend of Heroes:...