Setubal police commissioner Andreia Gonçalves described a dispute between four men aged between 30 and 60 and said one of them had killed the others before shooting himself.
She described the deaths as “an isolated situation” related to an unresolved issue between the men.
The four dead individuals were participating in a pigeon race, Portuguese media reported from the scene, and their disagreement also centred on an illegal vegetable garden.
Portugal has restrictive gun laws but firearms are legal for hunting. Of the 80 or so annual homicides that Portugal has registered on average since 2015, around a fifth are committed with firearms, according to police statistics.