Ryan McCoshan struck 63-year-old Peter Brown with the weapon, which had a 26-inch blade, outside his grandmother's house in Dumbarton.
Mr Brown suffered horrific injuries after the sword sliced into a bone in his arm.
He has been so badly affected he has been unable to work since the attack, last September.
At the High Court in Glasgow judge Lady Rae told McCoshan: "This was an appalling attack on a man in his 60s who was, at least initially, going about his business as a delivery driver.
"You caused him a very serious injury."
Lady Rae told McCoshan that, but for his early plea of guilty, she would have jailed him for six years.
McCoshan, 26, admitted assaulting Mr Brown to his permanent disfigurement and impairment.
Mr Brown was working for a Chinese takeaway in Dumbarton at the time of the attack last September 1.
Prosecutor Martin Macari said: "Mr Brown knocked the door, but received no answer.
"The door was answered by McCoshan.
"An argument took place over the length of time it had taken for the door to be opened."
The dad-of-one paid for the meal before he swore at Mr Brown.
Mr Brown confronted him about the comment and McCoshan then ran upstairs, returning clutching a samurai sword, and lashed out.
As the assault continued, Mr Brown's friend William Towie grabbed a nearby For Sale sign and charged at McCoshan to try and stop him.
McCoshan's 75-year-old grandmother Theresa also came out and hit her grandson to try and end the attack.
McCoshan was later found hiding in the loft of his house and arrested.
The court heard Mr Brown needed a blood transfusion and vital surgery. He is badly scarred and has nerve damage which could be permanent.