The Boston Marathon, now in its 121st year, begins Monday morning.
More than 260 people were wounded when the bombs exploded and sprayed shrapnel into the crowds.
While the monuments remain conceptual, city officials said they will be completed by next year's Boston Marathon and will precede a larger memorial to be erected by 2020.
Family members of the victims were on hand for the ceremony, standing alongside to pause and remember.
Top runners this year: Last year's champions are running.
Turkey votes 'Yes' in referendum, President Erdogan thanks people for support
The war in Syria led to some 3 million refugees crossing the border into Turkey. A state of emergency imposed in the coup aftermath remains in effect.
The fourth anniversary of the deadly Boston Marathon attacks was being marked on Saturday by Bostonians.
Richard stood Saturday with his wife, Denise, their daughter, Jane, and oldest son, Henry, in silence as bagpipers performed "Amazing Grace". Sean Collier, an MIT police officer, was shot and killed by the attackers later in the week.
Bomber Tamerlan Tsarnaev was killed in an ensuing shootout with police. His younger brother, Dzhokhar Tsarnaev, was convicted on federal charges and sentenced to death.
In 2015, Walsh declared April 15 to be One Boston Day, with the goal of taking a negative event and turning it into a positive one, CBS Boston reports.
The attacks have inspired several movies, including the Mark Wahlberg film "Patriots Day".
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