Prosecutors in Morelos state said the victims, ranging in age from 14 to 21, were found Thursday in plastic bags on a Cuernavaca street along with a threatening note from a drug gang. Photos from the scene showed a handless arm lying near the handwritten note.
The youths had apparently been strangled or asphyxiated before being dismembered, according to the prosecutors' statement.
Prosecutors said the four were students at local schools, though the mother of the youngest victim, 14-year-old Brandon Contreras Gutierrez, said the boy had left home 1 1/2 weeks before "and had been hanging out with others, driving around on minibikes" in one of the city's rougher neighborhoods.
It was not clear if he had any relationship with gang members who frequently use scooters or mini-motorcycles to distribute drugs or transport gunmen.
Morelos state Interior Secretary Oscar Hernandez Benitez said in a statement that he had ordered stepped-up patrols in the city following the killings and he called on the public "to form a united front against violence, for peace and tranquility in our state."
Local media reported that two high schools in the victims' neighborhood were briefly closed and evacuated after the bodies were found. The education department did not immediately respond to requests to confirm that.
Cuernavaca, nicknamed "The City of Eternal Spring," was once known mainly as a balmy resort area for tourists and as a weekend retreat for wealthy residents of Mexico City, just 40 miles (65 kilometers) away.
However it has become the scene of drug cartel turf battles in recent years, and the city was shocked when the son of poet Javier Sicilia and six friends were found dead, their bodies found stuffed into a car on March 28, 2011. A half dozen alleged drug gang members have been indicted in connection with those killings, and are facing trial.
Sicilia went on to found a nationwide movement advocating an end to Mexico's drug violence, which saw at least 47,715 people killed between December 2006, when President Felipe Calderon launched an offensive against drug cartels, through September 2011, the latest figures made available by the government. Thousands more are thought to have disappeared.
On Friday, Sicilia said the new slayings prove that little or nothing has changed in the year since his son died and that the government continues to fail to prevent or investigate such crimes.
"Unfortunately, we are seeing the same story over and over again," said Sicilia, "These crimes in Morelos, along with the crimes throughout the nation, are proof of the government's lack of effectiveness and its distance from the people."
"The clearest proof of that is that we continue to have 98 percent of crimes that go unpunished."
To address such concerns, Calderon unveiled the headquarters of what his government calls its "scientific police division," a wing of the federal police that will use specialized forensics and crime-scene techniques to boost investigations that in the past have been marked by clumsiness and poorly preserved evidence.
Calderon acknowledged the weaknesses of the past, saying police were "not very professional and in many cases incapable of fulfilling their primary mission of ensuring citizens' safety and, in some cases, they even allied themselves with criminals."
The new, 700-million-peso ($55 million) facility will have DNA, ballistics and fingerprint labs as well as a cyber-crime unit.
Calderon said that Mexico has made progress in detaining top drug traffickers, but that street-level violence between lower-ranking gang members has continued apace.
This week's killings in Cuernavaca bore the hallmarks of gang-on-gang violence, as do many of the killings in Cuernavaca, where victims' bodies have been hung from overpasses and left with hand-lettered signs.
The many Spanish-language schools in Cuernavaca that cater to foreign students have argued that violence is largely limited to those with gang links and say the city is still safe for the average person.
One such school, the Universidad Internacional, says on its website that "Cuernavaca is safer than most North American cities of its size."
"The violence is waged among themselves. Visitors and locals are safe!" wrote Virginia Ramos Foster, who leads study groups at the school.
Sicilia criticized such reasoning, even if many victims are gang members.
"If they are killing each other ... they still deserve justice," Sicilia said. "They were not born criminals, the government and society are not giving these youths what they need to become men, not criminals."