The men allegedly held up the family on their farm, Wolfskop, in Bainsvlei, outside Bloemfontein, on Wednesday around 19:00, Constable Maselela Langa said.
"The four suspects went into the house using a back door and they forced the father, mother, nephew, and son to a living room where they demanded money."
One of the robbers had a gun, the others knives. They could not find money but took a laptop computer, an iPad, three cellphones, and a handbag.
"The victims were not seriously injured during the incident," said Langa.
Police have asked anybody with any information about the robbery to contact their nearest police station.