Four years ago, I was on a journey, during which, I missed either Zhuhr or `Asr Prayer, I cannot recall which one now,
knowing that I abandoned it out of negligence and laziness. Now, I feel very sorry for committing this sin, and I seek forgiveness from Allah for every sin and misdeed I have ever committed, what should I do? Should I expiate this?
You should offer sincere Tawbah (repentance) to Allah, and you do not have to make up for this Salah (Prayer), for abandoning Salah intentionally is major Kufr (disbelief that takes the Muslim out of Islam) due to the saying of the Prophet (peace be upon him):
The covenant that distinguishes between us and them (the disbelievers) is Salah. Anyone who abandons it has committed Kufr (disbelief).
And his saying (peace be upon him):
Between a person and Shirk (associating others with Allah in His Divinity or worship) and Kufr is abandoning Salah.
Muslim in his Sahih.
(Part No. 6; Page No. 51) The expiation of this is fulfilled by offering sincere Tawbah.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.