The death of the man, 35, who has not been named, comes after three young men died in an eight-day period.
Police suspect all four fell victim to a Class A drug, probably heroin.
A West Yorkshire police spokesman said: "Four such deaths in such a short period and within such a localised area creates suspicion there may be a rogue batch of heroin on sale in the Bradford area."
'Enormous risk'
On Friday, 23 August, the body of a 24-year-old man was discovered at an address in the Clayton area.
Seven days later a 23-year-old man was found dead at his home in Ravenscliffe.
The following day a 21-year-old man died at an address in Scholemoor.
The police spokesman said: "Drug dealers are strongly urged to dispose safely of any remaining substances and drug users must realise the enormous risk to their lives when they use such drugs."
Detective Inspector Sheridan Moore, head of the district drugs team, said: "These deaths highlight the grave dangers of using illegal drugs.
"Drug users clearly do not know the content and purity of what they are taking and it goes to show how drugs are ruining lives and causing misery within local families.
"We would remind people about our "Dob in a Dealer'" campaign which urges people to expose dealers who are destroying lives, by ringing Crimestoppers anonymously with information."