"The demonstrators [in Kibera] were charging at the officers with stones and that is when police fired at them. Four of them have been killed," a police commander was quoted by the AFP news agency as saying.
At least 20 other Kenyans have been killed over the past two days.
Kenya has seen a wave of violence since Kibaki was elected in disputed presidential polls last month.
Friday was the last of three days of planned protests. Odinga's Orange Democratic Movement (ODM) said it was ending the protests because civilians were paying too heavy a price.
"Today is the last day of demonstrations. We have seen a lot of suffering caused by reckless police action against peaceful protestors," Salim Lone, the ODM spokesman, said.
But some protestors have vowed to return to the streets.
"We're very angry about this, and we will continue until Raila's in power," said Paul Oteyo, a 20-year-old from Kibera.
'Protecting lives and property'
A police official said one man was slashed to death by machetes in fighting in the Mathare slum located in Nairobi, the capital, overnight.
A police commander in western Kenya said another man had died in tribal fighting, also overnight.
In the town of Narok, Masai youths killed a member of Mwai Kibaki, the president's Kikuyu tribe with a poisoned arrow.
Patrick Wambani, a police commander, said: "It is unfortunate that he met his death when he strayed into a group of armed Masai youths who were targeting members of the community in the area."
Seven deaths were reported in the western opposition stronghold of Kisumu and the capital Nairobi, mainly in volatile slums where hundreds of protesters clashed with police, Wambani said.
Eric Kiraithe, a police spokesman, said on Friday that security forces would not allow opposition supporters on the streets for further demonstrations.
He said: "Police will be steadfast in protecting lives and property."
In the past two days, police have fired live ammunition and tear gas and beat protesters with sticks.
Nine western governments, including Australia, the UK and Canada, issued a statement on Friday urging Kenyan government forces to stop killing unarmed civilians.
"We urge security forces to exercise their duties strictly within the boundaries of law and desist from any extraordinary or disproportionate use of force and, in particular, the killing of unarmed protestors," the statement said.
Prior to the nationwide protests that began Wednesday, more than 700 people died in tribal killings and clashes with police and around 260,000 people have been displaced.