All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
This woman has no right to charge her father in-law with Zina (fornication or adultery) with her 4-year-daughter, nor to assault him unless she produces Sharee’ah proof, which is four witnesses.
Allah Says (what means): {And those who accuse chaste women and then do not produce four witnesses - lash them with eighty lashes and do not accept from them testimony ever after. And those are the defiantly disobedient.} [Quran 24: 4]
Allah also Says (what means): {Why did they [who slandered] not produce for it four witnesses? And when they do not produce the witnesses, then it is they, in the sight of Allah, who are the liars.} [Quran 24: 13]
Thus, charging him without evidence makes her liar. So, she has to be patient and to seek Allah's reward. However, if she is right that he sexually abused her daughter, then Allah will take revenge on her behalf.
Allah knows best.