The Bluebirds had entered this game third in the npower Championship, a point behind Norwich with both sides having two games to play.
But their hopes of putting pressure on the Canaries were dashed as a woeful first-half display saw Boro open up a 3-0 lead inside 21 minutes as Leroy Lita's 99th career goal and further strikes from Barry Robson and Richard Smallwood put Tony Mowbray's men in total control.
The result condemned Cardiff to their heaviest home loss since their final match at Ninian Park against Ipswich in April 2009, and meant a win for Norwich in their late game at Portsmouth - or, failing that, at home to Coventry on Saturday - would seal their promotion at the expense of the Bluebirds.
Cardiff look set to face another play-off campaign, unless the FA elect to mete out a heavy punishment to QPR over their signing of Alejandro Faurlin when the outcome of that disciplinary process is announced later this week.
Source: PA
Source: PA