Japan on Wednesday commemorates the fourth anniversary of the quake-tsunami disaster which swept away thousands of victims and sparked a nuclear crisis, while survivors are still struggling to recover from the tragedy.

Remembrance ceremonies will be held in towns and cities around the disaster zone and in Tokyo, where Emperor Akihito and Empress Michiko are to lead tributes to those who died in Japan's worst peace-time disaster.

A national moment of silence is to follow the cry of tsunami alarm sirens which will be set off at 2.46 pm (0546 GMT), the moment a 9.0-magnitude undersea quake hit.

Its gigantic force unleashed a towering wall of water that travelled at the speed of a jet plane to the coast. Within minutes, communities were turned to matchwood, and whole families drowned.

The National Police Agency said a total of 15,891 people are confirmed to have died in the disaster with another 2,584 still listed as missing. Human remains are sometimes still found years later.

Thousands of people afflicted by the tsunami remain unable to turn the page on the horror of 2011. For them, grief is frozen in time because they cannot find the bodies they need to mourn.

"Somebody needs to do this, walking along the shore," said Takayuki Ueno, who combed a desolate winter beach during the weekend for the bones of his three-year-old son killed by the monstrous tsunami.

"Otherwise there is no possibility that anyone will be found," said the 42-year-old man whose daughter and parents were also killed by the tsunami in northern Fukushima.

Massive waves also crashed into the Fukushima nuclear plant, triggering reactor meltdowns and explosions, and setting off the worst atomic crisis in a generation.

The crippled plant remains volatile and the complicated decommissioning process is expected to last for decades, as fears persist over the health effects of leaked radiation. Tens of thousands were evacuated from the stricken area.

Despite the government pledging billions of dollars in reconstruction aid, progress in disaster-hit regions has been slow, some communities remain ghost towns, and thousands of disaster refugees struggle to cope.

According to the government, more than 80,000 survivors still live in temporary houses, while nearly 230,000 people, mostly evacuees from Fukushima, are taking refuge.

"Reconstruction is shifting to a new stage," Prime Minister Shinzo Abe told a news conference on Tuesday.

"We will help disaster victims become self-sustaining," Abe said. "As the government, we will provide the best possible support."