Jason Edward Norman, 45, from Merthyr Tydfil, was jailed for 16 months at Cardiff Crown Court after admitting causing death by dangerous driving.
Andre Murphy, 56, was hit on 14 December 2015 while changing a tyre on a horse trailer near junction 28.
Mr Murphy, known as Nick, from Wexford, Ireland, was on his way to Surrey when the crash happened.
Norman crashed into Mr Murphy at 54mph (86km/h) close to Tredegar Park roundabout in Newport.
Prosecutor Claire Wilks told the court: "The transporter had its hazard lights on and Mr Murphy placed a traffic cone 50 yards down the road when the Renault Scenic collided with him."
In a victim impact statement read to the court, Mr Murphy's partner Julie Redden added: "I have lost my lovely, lost my friend and my soul mate."
Norman was also banned from driving for five years.
A man has been jailed after killing a stud farmer in a crash as he changed a flat tyre on an M4 slip road.