Police suspect a sharp weapon could have been used in the crime.
The deceased are identified as Shardaben Gohel (35), her son Rutvik (13), and two daughters Doli (12) and Neha (7), a police official said, adding that Shardaben Gohel used to work as a farm labourer along with her husband, who had gone to work this morning.
"Bodies of Shardaben Gohel, Rutvik, Doli, and Neha were found in a septic tank of a house allotted to them by the owner of the farm," Mangrol police station sub-inspector H V Rathod said.
He said police have registered a case of murder on the complaint lodged by Shardaben Gohel's father Devabhai Mewada, and further investigation is on.