Radak: (a) It discusses four who go from affliction to relief, and they must thank Hashem, for He relieved them ¹ . It is not chance, like errant researchers say that man is under Mazal. Rather, the affliction was punishment for their sin, and through Hashem's Chesed they were saved from it when they called to Him. (b) One who had good in his place, Hashem brings him to lack, and one who was in a bad state, Hashem elevates him and saves him, and gives to him great good. All is from Him - good and bad.

Malbim: After it mentioned the gathering of exiles [in the previous Mizmor], it says that those who return from Galus will offer Korbanos Todah for all four matters for which one must thank.

Malbim: Chesed is what one does without any prior obligation or promise, and even if the recipient is not worthy based on his deeds. Regarding Hashem, Chesed refers to a miracle above nature.