Is it necessary for a traveler to recite the prayer for travel upon returning from a country where he stayed for two or three days, or is that prayer specific to one who stayed for more than four days?

It is prescribed for every person returning from a journey to recite the supplication for travel. Scholars have not differentiated between long and short journeys, as every journey involves a return. All of them narrate from Ibn 'Umar that the Prophet (peace and blessings be upon him) whenever he returned from a military expedition, Hajj, or Umrah, would utter three takbīrāt upon every elevated spot on the earth. Imam An-Nawawi mentioned that the Sunnah is for the person returning from their journey to say: "آيِبُونَ تائِبُونَ عابِدُون لِرَبِّنا حامِدُونَ" (We are returning, repenting, worshipping, and praising our Lord). Al-Munawi said: "This dhikr (remembrance) is sunnah for every journey undertaken for an act of obedience, and even for permissible journeys. Abu Zur'ah even extended it to forbidden journeys, because the one committing a forbidden act is more in need of dhikr than others." As Al-Sa'di said in Bahjat Qulūb Al-Abrār: "We ask You, O Allah, to make us, in our return and coming back, steadfast in repentance to You, in Your worship, and in Your praise, and to conclude our journey with Your obedience, just as You began it with the success therein."