The fire was reported shortly before noon in a residential neighbourhood of Rupperswil, a village between Aarau and Lenzbourg.
Based on the findings of the cantonal forensic service all four people who had been in the house at the time of the blaze were homicide victims.
Police initially would not confirm how many bodies were found in the home, adding that they were badly charred.
But further investigation revealed the number of victims and the fact they had suffered cuts and punctures, the prosecutor's office said.
Police believe the fire was set intentionally.
To clarify the exact circumstances of the victims' deaths, the prosecutors' office has ordered autopsies, which were to be carried out on Tuesday and Wednesday.
Police had not established the identities of the people killed.
A woman who had separated from her husband lived with her two children in the home and had a boyfriend, the 20 Minuten newspaper reported, citing neighbours.
The prosecutor's office, meanwhile, has asked witnesses who may have information with regard to suspicious activities around the house between Sunday evening and Monday morning to contact Aargau cantonal police.
Police have interviewed witnesses who saw a brightly coloured subcompact car nearby.