The spike has prompted an urgent health warning from police and paramedics about the risk of taking illicit substances.
Two men, both aged 39, and two women, aged 26 and 30, are dead.
Six more people have presented to hospitals after overdosing on a combination of alcohol and illicit drugs.
Paramedics are finding people collapsed after drinking and taking either heroin, amphetamine or fantasy, Queensland Ambulance Service clinical director Tony Hucker said.
“When you combine the two that’s a higher risk combination,” he said.
He said that didn’t mean taking drugs on their own is safe, but added people can make safer choices that minimise the risk.
“Obviously to manage the risk is don’t take drugs, but we understand the reality out in the community,” Mr Hucker said.
“If you’re going to take one of these drugs, take half a dose when you start, make sure you don’t re-dose in the first 90 minutes, and always make sure you have friends with you that are prepared to look after you.”