In the Indonesian province of Aceh, where more than half the victims of the tsunami were killed, prayers were said and local communities gathered on beaches to mourn the dead.
"Even after four years, I cannot forget how I lost hold of my wife and baby," Ibrahim Musa, a 42-year-old civil servant, told the Associated Press.
"I have tried in vain to look for them for three years. Now I have no choice but to accept their departure as destiny."
In southern Thailand, where more than 5,000 people died, many of them tourists, family members will light candles and a ceremony was to be held in honour of the hundreds of victims who remain unidentified.
Homeless
The tsunami devastated communities across 12 countries, but four years after the disaster hundreds of thousands of homes, schools, hospitals and businesses have been rebuilt in the largest relief operation ever seen.
Despite the billions of dollars ploughed into reconstruction efforts, thousands of families remain homeless or in poor temporary shelters.
In Sri Lanka, where 31,000 people died, more than 10,000 survivors remain in squalid temporary camps.
Waste
In 2005, the Sri Lankan government's auditor general said only 13.5 per cent of the $1.16bn committed to assist victims had been spent. There have been no government audits released since.
Problems of waste and bureaucratic mismanagement were underscored in October when the government destroyed more than five tonnes of rice and lentils donated by the World Food Programme, as it rotted before it could be distributed.
Mismanagement has also tainted the much smaller aid effort in Malaysia, where 68 people died in the disaster.
Malaysian government auditors have reported mishandling of aid money that ended in shoddy houses or fishing boats unsuitable for local waters.