The fourth series of the British police procedural television programme Line of Duty was broadcast on BBC One between 26 March and 30 April 2017. It is the first series to air on the network after the first three were broadcast on BBC Two.

Line of Duty is a British police procedural and serial drama created by Jed Mercurio and produced by World Productions for the British Broadcasting Corporation (BBC). It first began broadcasting on BBC Two on 26 June 2012. The programme performed well and was quickly commissioned for additional series that aired in 2014 and 2016. After becoming the highest-rated series on BBC Two in 10 years, Line of Duty was promoted to BBC One beginning with the fourth series in 2017 and fifth in 2019, securing commissions through a sixth series, which concluded on 2 May 2021, after the programme had aired a total of 36 episodes.