Salaam Abu Hafsat,
Your citing of Surah Nisa'i verse 65 (4:65) as evidence/prove that we are required to follow and uphold other source besides the Quran is misleading, for two reasons.
The first, and main reason, why it is misleading is because Allah has already instructed us (including the prophet) to judge ONLY by/with the Quran, as evident by the following verse:
"And We have revealed to you, [O Muhammad], the Book in truth, confirming that which preceded it of the Scripture and as a criterion over it. SO JUDGE BETWEEN THEM BY WHAT ALLAH HAS REVEALED and do not follow their inclinations away from what has come to you of the truth. To each of you We prescribed a law and a method. Had Allah willed, He would have made you one nation [united in religion], but [He intended] to test you in what He has given you; so race to [all that is] good. To Allah is your return all together, and He will [then] inform you concerning that over which you used to differ." [Quran 5:48]
So, with respect to the Surah and verse you cited (4:65) in which the prophet is being instructed to judge between their disputes, it is clear, as per the above surah and verse which I cited (5:48), that he is expected to judge ONLY by what Allah has reveled to him (which is the Quran).
It is common knowledge, among all those who have read and studied the Quran, that one of the names used for the Quran is 'Al-Furqan; which means the criterion between right and wrong. Which only further supports the claim that the prophet would have used it (the Quran ONLY) to judge his community.
HOW THE QURAN DESCRIBES ITSELF
http://quransmessage.com/articles/how%20the%20quran%20describes%20itself%20FM3.htmThe Qur’an was Muhammad’s only Revelation
http://www.detailedquran.com/quran_data/The_Only_Revelation.htmThe second reason why your usage of 4:65 as evidence/prove that we are required to follow and uphold other source besides the Quran is misleading is for the simply fact that the prophet is no longer with us. He is dead. So how can we take our disputes to a dead person? But if we accept the Quran sufficient for us as a judge, then your whole argument flies out the window.
You also said:
"This Ayat is an answer to those who claim that it is sufficient for them in their religion to follow the Book of Allah only..."
Allow me to ask you a question: Why shouldn't the Quran be sufficient enough for us to follow? How can you say that the Quran, which makes the following claims, "is not sufficient for us"?:
The Quran claims to be a fully detailed explanation (Arabic: fussilat 41:3; 11:1)
It claims to be a guidance without a doubt (2:2-3).
It claims to be the perfect guidance (Arabic: hudan 2:2).
It claims to be a clear convincing proof (Arabic: burhan (4:174).
It claims to be the ultimate scale (Arabic: mizan, 42:17; 57:25).
It claims to be the discernment between truth and falsehood (Arabic: Furqan 25:1, 2:53).
It claims to be an evidence absolutely clear (Arabic: bayyina 20:133).
It claims to be a clear explanation of all matters (Arabic: tibiana lekulli shayin 16:89).
It also claims to be the only source which guides to a path which is most firm, right, straight and stable (6:114; 17:9; 16:89).
Please, also, ponder over the following verse:
“ISN’T IT SUFFICIENT FOR THEM THAT WE HAVE REVEALED TO YOU the Book which is recited to them? Indeed, in it are blessings and admonition for the Believers.” [Quran 29:51]
And if, as you calim, the Quran isn't sufficent enough for us as a guide, then what purpose does it surve?
http://quransmessage.com/articles/quran%20sole%20guidance%20FM3.htmTHE QURAN STANDS ALONE AS SOLE RELIGIOUS GUIDANCE
Further more, you also said:
"without following the Sunnah of His Messenger."
Any and all religious assertions or theological standpoints/practices must find unequivocal support from the Quran; any theological standpoint whatsoever in the name of God's religion, must provide clear, unambiguous, unequivocal proof and authority from the Quran for it; as it is the Quran that is the primary and ultimate authority from God.
That being said, please provide CLEAR, EXPLICIT, UNAMBIGUOUS and UNEQUIVOCAL proof/verses from the Quran which make it incumbent upon Muslim to follow the so called 'Sunnah of Muhammad' (if that Sunnah is anything but the Quran).
Finally, allow me to post to you a question:
which version of the sunnah are you refering to? The Shia or the Sunni version? Why one and not the other? In other words, there is no single definition of Sunnah that is acceptable to all the Islamic scholars. Some say that whatever the Prophet (PBUH) said or did in private, or public, is his Sunnah. Others say that whatever is in the books of hadith is the Sunnah of the Prophet (PBUH). Still others disagree with this definition of Sunnah and say that what the Prophet (PBUH) did as a Bashar (man) is not his Sunnah; and only the things he did as a prophet comprise it. Therefore, is safe to conclude, then, that there are as many variations in the definition of Sunnah as there are sects and sub-sects in Islam. Each sect thinks its definition is the right one.
UNDERSTANDING THE TERM 'SUNNA' FROM A QURAN'S PERSPECTIVE
http://quransmessage.com/articles/understanding%20the%20sunna%20from%20a%20quran's%20perspective%20FM3.htmPeace.