Two adults and two children were killed when a refugee boat capsized off the coast of Lesbos, Greek authorities said on Wednesday.
Seven others were rescued.
The Greek coastguard on Wednesday also picked up another 90 migrants travelling on dinghies between Turkey and the island of Lesbos, an officer said.
Hundreds of thousands of migrants arrived on Lesbos from Turkey in 2015 and in the first three months of 2016.
But the numbers have declined since March, when the European Union struck a deal with Ankara to stem the rate of arrivals.
Officials on the island said that between Tuesday morning and Wednesday morning 44 migrants arrived on Lesbos after setting sail from Turkey.
In February, nearly 2,000 people were arriving on average every day.
Police in northern Greece said they have broken up several people smuggling rings in recent days, including a group of 29 individuals near the city of Thessaloniki. A Greek policeman was among those involved in the smuggling operation, authorities said.
Smuggling groups have been seeking out new routes of illegal entry in northern Greece, including along the Greek-Turkish border on the river Evros, to avoid the coastguard and international patrols.
Police also said they arrested a number of smugglers near Thessaloniki who had been holding five migrants captive in containers to extort money from relatives.