Vasilijs Ransevs, 33, was found dead from a head injury in his bedsit in Portland Street, Lincoln, on 16 June.
A 17-year-old girl and three men, aged 20, 29 and 31, were arrested earlier after police executed warrants at several addresses across the city.
They are all helping police with their inquiries and officers are still appealing for information.
Det Supt Stuart Morrison said: "We are still appealing for information in this case and we would urge anyone who knows anything about Mr Ransevs and his death to get in touch with us as soon as possible."
Mr Ransevs had lived in Lincoln for about two years, and in Peterborough prior to that.
Police said a friend found him dead in the bedsit, and he had been last seen alive on 14 June.
Police have arrested four people in connection with the death of a Latvian agricultural worker.