Dylan Mackie covered the “defenceless” youngster in bruises from head to toe.
A medical expert called them “some of the most extensive non-accidental injuries he had even seen”.
Mackie, a pressure-washing operative, had been left in charge of the boy, who was “completely uninjured” when the child’s mum left for work on the morning of April 16 last year.
Neighbours then heard banging and a voice saying, “stop it, stop it” from the flat in Grangemouth where Mackie was supposed to be watching the child.
Mackie (22),of Kersiebank Avenue, Grangemouth, appeared for sentence at Falkirk Sheriff Court today having been found guilty following a three-day trial last month.
The boy, still just five-and-a-half, ws the principal prosecutino witness, giving evidence by video from a side room.
He said: “Dylan hit me. He hurt my face. Dylan did it in my bedroom with his hands.”
The court heard Mackie had later taken the child with him when he went to work himself.
The boy was asked what Mackie did at work.
He replied: “Hitting me.”
The boy’s 23-year-old mother described her son as looking “like a battered wean” when she undressed him for his bath that evening.
Sheriff Craig Caldwell told Mackie: “These injuries, principally and most seriously to his head and face, also covered his entire body, including his back, his bottom, his arms, legs and torso.
“They were the result of a sustained beating which you offered this defenceless four-year-old child.
“It’s due to no restraint on your part that they were not life-threatening, or resulting in scarring, but there will be, inevitably in a child of that age, psychological injuries as well.
“You were in a position of trust, and given all these circumstances there is no alternative but to impose a custodial sentence.”
The sheriff also made Mackie subject to a 12-month supervised release order.
During the trial, jurors were shown what Sheriff Caldwell described as “graphic” photographs of the boy after the beating, including “a significant facial injury”, bruising to his left ear, bruising behind both ears, and extensive bruising to his buttocks.
They were also shown a partially-burned plank found by police at the rear of the Grangemouth flat.
Consultant paediatrician Michael Colvin said the shape of the charred end of the plank fitted the shape of the injury on the side of the boy’s head, but no forensic evidence could be find to link it to the crime.
Dr Colvin said the injuries were “some of the most extensive non-accidental injuries” he had ever seen on a child.
Mackie denied assaulting the boy “with his hands and other means unknown”, to his severe injury.
He said he had struck the boy on the buttocks after he soiled himself, but insisted the injury on his head must have been caused by a fall.
He also denied assaulting the boy’s mother in an incident three days later, pulling her pony-tail, causing her to flee their flat “screaming for help, terrified, distressed, and barely able to speak”.
The jury of nine women and six men took less than an hour to find Mackie guilty of both charges.
Simon Hutchison, defending, said Mackie had “continued to maintain his innocence” while discussing his conviction with social workers preparing pre-sentencing reports.
He said: “Of course the difficulty with that is that it leave the court with little options available in terms of sentencing.
“What I can say, quite legitimately, is that he has been found guilty of something that is entirely out of character for him.”
Mackie showed no emotion as the sentence was pronounced.