A fifth Palestinian was reportedly killed later in the day by an Israeli tank.
The Israeli army has been carrying out near daily air and ground offensives against Palestinian factions in the Gaza Strip saying they are attempting to halt the rocket and mortar attacks.
However, there had been a lull in violence last week as both sides sought a truce, through Egyptian mediation, after an Israeli incursion that left around 120 Palestinians dead and an attack on Jewish seminary in Jerusalem.
The latest raids came as the armed wing of Hamas said it was now able to target Israeli aircraft using weapons abandoned by security forces loyal to Mahmoud Abbas, Palestinian president, when they were pushed from Gaza last year.
"We consider the targeting of Israeli aircraft to be a new development because certain weapons became available to us after the military stabilisation," Abu Obeida, a spokesman for the armed wing of Hamas, told the AFP news agency.
"These weapons were in the arsenals of the Authority and were supplied to them by foreign countries," he said, referring to Abbas's Palestinian Authority.
Israeli television said on Friday that an army helicopter had been hit by Palestinian fire as it was flying over Gaza, the first incident of its kind in several years.
Abu Obeida would not specify what weapons were used against the helicopter, but Hamas-run TV in Gaza has broadcast footage of fighters firing heavy machine-guns that could be used against aircraft.
The Israeli army has declined to comment on the incident.