The evidence of a 'ploughed field' at the Harappan site of Kalibangan is an important archaeological discovery that provides insights into the agricultural practices of the Harappan civilization. This discovery indicates that the people at Kalibangan were engaged in agriculture and used ploughs for cultivation. The presence of a ploughed field is a significant piece of evidence that helps archaeologists and historians understand the agricultural techniques and technology of the Harappan civilization.

So, the correct option is [4].