To what extent is the hadith "May Allah have mercy on a person who prays four rak'ahs before Asr" authentic, and is it a supplication from the Prophet, peace and blessings be upon him, that includes whoever observes these rak'ahs even once, or is continuous observance required?

The Hadith, "May Allah have mercy on a person who prays four rak'ahs before Asr," was graded as hasan (good) by Tirmidhi and authenticated by a number of contemporary scholars. This is evidence for the desirability of praying four rak'ahs before the Asr prayer, but they are not among the emphasized regular Sunnah prayers. These rak'ahs are prayed two by two (i.e., two rak'ahs then two rak'ahs), and doing so is an act of drawing closer to Allah and obedience. It is hoped that whoever prays them will receive Allah's mercy. The meaning of "May Allah have mercy on" could be a supplication or an affirmation that Allah will have mercy on the doer.