Craig Tilbury, 25, from Laurencekirk, Aberdeenshire, admitted the culpable homicide of James Simpson at Public Street, Blairgowrie, on June 16 by punching him on the head and knocking him to the ground.
The High Court in Glasgow heard Tilbury contacted police and confessed before the paramedics had arrived to treat Mr Simpson.
Sentencing Tilbury Judge Norman Ritchie QC said: "Mr Simpson was out celebrating Father's Day. He had a little more alcohol than was wise."
The court heard that there was an earlier altercation in which Mr Simpson swung a punch at Mr Tilbury.
The helicopter engineer refused to let in lie and went outside to where Mr Simpson was standing waiting for a bus and hit him once on the head causing him to fall to the ground.,This caused a freak injury to Mr Simpson which resulted in his death.
Judge Ritchie told Mr Tilbury: "You approached Mr Simpson without warning and as he turned you hit him and he fell to the ground.
"What is remarkable in this case is that after that criminality you realised the enormity of what you did and got in touch with the police even before the paramedics got to the scene.
"I take into account contrition and your remorse which is clearly genuine."