Aged between five and 13, they appeared to have drunk a bottle of pesticide, officials say. Police have not ruled out suicide.
The parents of the three girls and a boy had reportedly left the village, near Bijie city, in search of work.
The area is one of the poorest in the country and has seen previous deaths of abandoned children.
Chinese state media report that the mother of the four siblings left three years ago, followed by their father in March.
The children had reportedly dropped out of school a month ago, according to the Xinhua news agency. It added that their only food was corn and preserved meat.
The mass urban migration of Chinese parents looking for work has led to millions of children being left behind in villages; many of them are cared for by grandparents.
Correspondents say that such children can be highly vulnerable.
Three years ago there was a national outcry after five abandoned children died in Bijie.
The children had suffocated inside a rubbish bin where they were sheltering, after apparently lighting a fire to keep warm.
Chinese officials have said that at least 80 million people live below the poverty line, surviving on less than $1 (Â£0.65) a day.

Cezary Sobiewski, 40, murdered Barbara Baraniecka weeks after she confided concerns about his drinking to friends.
A judge said Sobiewski went to buy cider after the "sustained and brutal attack" at the couple's home in Tipton, West Midlands.
Sobiewski admitted Ms Baraniecka's murder on Monday.
He entered the plea on what would have been the start of his trial at Wolverhampton Crown Court.
Updates on this and other stories in Birmingham and the Black Country
The Crown Prosecution Service (CPS) said Ms Baraniecka had told Sobiewski she wanted to end their relationship last October, a month before her death.
She had already confided in friends she had concerns about his increased alcohol consumption.
Police said Sobiewski, who had moved to the UK from the US in 2010 to live with Ms Baraniecka, was arrested at the scene.
Her 25-year-old daughter found her covered in blood on her bed as Sobiewski sat in the kitchen.
Paramedics confirmed she had been dead for several hours.
Sentencing him, Judge John Wait said: "Having killed Barbara Baraniecka in the course of a sustained and brutal attack, you went out to the supermarket, bought cider and sat at home drinking it, leaving Barbara's daughter to find her mother's body and call the emergency services.
"You took the life of someone who had many friends and who was the mother of two daughters."
Speaking after Sobiewski's guilty plea on Monday, Det Insp Jim Munro said: "I very much hope that now this case is over, it will in some small way allow Barbara's daughters to start rebuilding their lives after the terrible events of November last year."
"Cezary Sobiewski could not control his drinking or his jealousy and his actions have resulted in the death of a completely innocent woman."

Khalid Mohamed Omar Ali, 27, was arrested on suspicion of offences under the Terrorism Act and possession of offensive weapons on 27 April.
He was detained as part of an intelligence-led operation after a concerned family member is believed to have contacted the police.
Police said he remains in custody and can be questioned until 11 May.
It comes after a warrant of further detention was granted at Westminster Magistrates' Court on Wednesday.
Mr Ali is thought to be a UK national - but born overseas - and to have gone to school in Tottenham, north London.
He was arrested in Parliament Street, at the junction with Parliament Square, following a stop and search as part of an ongoing operation, police said.
Police are investigating the apparent pesticide poisoning of four abandoned siblings in rural south-west China.
[NEXT_CONCEPT]
A "jealous" man who stabbed his partner 13 times because he could not accept their relationship was over will serve a minimum of 20 years in prison.
[NEXT_CONCEPT]
Police have been granted another week to question a terror suspect arrested near the Houses of Parliament.