Is a son taking money from his father without his knowledge considered stealing, and is it obligatory to tell him about it after more than 4 years have passed, especially since the son is currently unemployed and the amount taken does not hold value for his father now?

It is not permissible for a son to take his father's money without his permission. If he does so, he must repent and return the money, unless the father absolves him. The father may or may not inform others of what his son has done, depending on what serves the best interest. However, if the father is not fulfilling his obligatory financial support (nafaqah), then it is permissible for the son to take what suffices him and his father with discretion, without needing to return it, based on the hadith of Hind bint Utbah.