Monday's attacks are the latest outbreak of post-war violence since the weak interim government, backed by Ethiopian troops, ousted the Union of Islamic Courts fighters in a two-week war.
Mohamed Nur, a resident in the area of the capital, said: "In one house, a man and his son died while his wife and daughter were injured. They have been rushed to hospital.
"Another woman died in our neighbourhood and at least three other people were wounded."
A Reuters TV witness said the single-room iron shack where the father and son were killed had been obliterated.
There have been almost daily gun, grenade and mortar attacks, most of them in the chaotic capital.
On Sunday, an explosion in the southern port of Kismayu killed at least four people and wounded about 25 others.
The interim government has blamed the bloodshed on Union of Islamic Courts fighters and foreign gunmen.
