Although Imam Ahmad Ridha (rahimahullah) gave fatwa not to kiss graves of awliya and stand about 4 feet away. However there is valid difference of opinion about kissing grave of Prophet (Peace be upon him) and Awliya. 

Fatwa of Imam Ahmad bin Hanbal (Rahimahullah). 

ﺳﺄﻟﺘﻪ ﻋﻦ اﻟﺮﺟﻞ ﻳﻤﺲ ﻣﻨﺒﺮ اﻟﻨﺒﻲ ﺻﻠﻰ اﻟﻠﻪ ﻋﻠﻴﻪ ﻭﺳﻠﻢ ﻭﻳﺘﺒﺮﻙ ﺑﻤﺴﻪ ﻭﻳﻘﺒﻠﻪ ﻭﻳﻔﻌﻞ ﺑﺎﻟﻘﺒﺮ ﻣﺜﻞ ﺫﻟﻚ ﺃﻭ ﻧﺤﻮ ﻫﺬا ﻳﺮﻳﺪ ﺑﺬﻟﻚ اﻟﺘﻘﺮﺏ ﺇﻟﻰ اﻟﻠﻪ ﺟﻞ ﻭﻋﺰ ﻓﻘﺎﻝ ﻻ ﺑﺄﺱ ﺑﺬﻟﻚ 

Translation: Abdullah bin Ahmad bin Hanbal asked Imam Ahmad (rahimahullah) about that person who touches the pulpit of Prophet (Peace be upon him) and SEEKS TABBARUK DUE TO TOUCHING IT. ALSO KISSES IT AND DOES THE SAME ABOUT BLESSED QABR (OF PROPHET PEACE BE UPON HIM) AND SEEKS TO DRAW CLOSER TO ALLAH THE EXALTED WITH THIS ACTION? Imam Ahmad (rah) said: THERE IS NO HARM IN IT. 

Kitab al ILal of Abdullah bin Ahmad bin Hanbal 2/492 # 3243 

Although in other references It is also narrated that Imam Ahmad bin Hanbal did not approve it. Ibn Hajr al-Asqalani (rah) also said that some of Hanblis ruled out the authenticity of this Imam Ahmad's qawl.  Ibn Hajr said right next to it. 

ﻭﻧﻘﻞ ﻋﻦ ﺑﻦ ﺃﺑﻲ اﻟﺼﻴﻒ اﻟﻴﻤﺎﻧﻲ ﺃﺣﺪ ﻋﻠﻤﺎء ﻣﻜﺔ ﻣﻦ اﻟﺸﺎﻓﻌﻴﺔ ﺟﻮاﺯ ﺗﻘﺒﻴﻞ اﻟﻤﺼﺤﻒ ﻭﺃﺟﺰاء اﻟﺤﺪﻳﺚ ﻭﻗﺒﻮﺭ اﻟﺼﺎﻟﺤﻴﻦ ﻭﺑﺎﻟﻠﻪ اﻟﺘﻮﻓﻴﻖ 

It is narrated by Ibn Abi Sayf al Yamani (rah) one of the Shafi'i scholars of Makkah, he considered it "ALLOWED" to "KISS THE MUSHAF OF QUR'AN, BOOKS OF HADITH AND GRAVES OF SALIHEEN" and (this is) with the Tawfiq of Allah. (Fath ul Bari 3/475) 

Imam al Zarqani also said about it. 

ﻭﻧﻘﻞ ﻋﻦ اﺑﻦ ﺃﺑﻲ اﻟﺼﻴﻒ اﻟﻴﻤﺎﻧﻲ اﻟﺸﺎﻓﻌﻲ ﺟﻮاﺯ ﺗﻘﺒﻴﻞ اﻟﻤﺼﺤﻒ، ﻭﻗﺒﻮﺭ اﻟﺼﺎﻟﺤﻴﻦ 

It is narrated by Ibn Abi Sayf al Yamani (rah) ash-Shafi'i, he considered it "ALLOWED" to "KISS THE MUSHAF OF QUR'AN AND GRAVES OF SALIHEEN" (Sharhal Zarqani ala Muwatta 2/458) 

al-Dhahabi (rah) emphatically approved kissing grave of Prophet (Peace be upon him). 

Here is his full quote: 

أَخْبَرنا أَحمد بن عبد الـمنعم، غير مرّة، أَنا أَبو جعفر الصيدلانـي ـ كتابةً ـ أَنا أَبو علـي الـحدَّاد ـ حُضوراً ـ أَنا أَبو نُعَيْـم الـحافظ، نا عبد اللّه بن جعفر، ثنا مـحمّد بن عاصم، نا أَبو أُسَامة عن عُبَـيْد اللّه عن نافع عن ابن عمر: أنَّه كَانَ يَكْرَهُ مَسَّ قَبْر النَّبـيّ .

قلْتُ: كَرِهَ ذلك لأنَّهُ رَآه إسَاءَةَ أَدَبٍ. وقد سُئِل أَحمد بن حنبل عن مَسّ القَبْر النَّبَوي وتقبـيـلَهُ فلـم يَرَ بذلك بَـأْساً، رَواه عنه وَلَدُه عبد اللّه بن أَحمد. فإن قـيـل: فهلاَّ فَعَلَ ذلك الصَّحَابَة؟ قِـيـل: لأنَّهُم عَايَنُوه حَيّاً وَتَـمَلّوْا به وقَبَّلُوا يَدَه وكادُوا يَقْتَتِلون علـى وُضُوئِه واقتَسموا شعرَه الـمُطَهَّر يَوْم الـحَجّ الأَكْبَر، وكان إذا تنـخّم لا تكاد نـخامته تقع إلاّ فـي يد رجلٍ فـيدلك بها وَجْهه، ونـحن فَلـمَّا لَـمْ يَصُح لَنَا مِثْل هذا النَصِيب الأوْفَر تَرَامَيْنَا علـى قَبْره بـالإِلتزام والتَّبْجِيـل والإِسْتِلاَم والتَّقْبِـيـل، أَلاَ ترى كَيْفَ فعل ثابت البُنَانـي، كان يقبّل يَدَ أنس بن مالك ويضَعُها علـى وجْهِهِ ويقول: يَدٌ مَسَّتْ يَد رسول الله . وهذه الأُمُور لا يُحَرّكُها مِن الـمسلـم إلاّ فَرْط حُبّه للنّبـيّ ، إذْ هُو مأمُور بأن يُحبَّ الله ورَسُوله أشَدَّ مِن حبّه لِنَفْسِه

وَوَلَدِه والنّاس أجْمعين، ومن أمواله ومن الـجَنّة وحُورِها، بَلْ خَـلْقٌ من الـمؤمنـين يحبُّون أبـا بكر وعُمر أكثر من حُبّ أنْفُسِهم 

Translation: Ibn Umar (RA) disliked touching the grave of Prophet (Peace be upon him) 

I (al-Dhahabi) say: He considered that disliked as he saw it to be "AGAINST ADAB (MANNERS)" and (indeed) Imam Ahmed bin Hanbal (rah) was asked about touching the grave of Prophet "AND KISSING IT" he said: I do not see "ANY HARM IN THAT" this was narrated from him by his son Abdullah bin Ahmed. If it is asked: "Why did the Companions not do this?" We reply: "Because they saw him with their very eyes when he was alive, enjoyed his presence directly, kissed his very hand, nearly fought each other over the remnants of his ablution water, shared his purified hair on the day of the greater Pilgrimage, and even if he spat it would virtually not fall except in someone's hand so that he could pass it over his face.Since we have not had the tremendous fortune of sharing in this, "WE THROW OURSELVES ON HIS GRAVE" as a mark of commitment, reverence, and acceptance, even to kiss it. Do you not see what Thabit al-Bunani (رضئ اللہ تعالی عنہ) did when he kissed the hand of Anas ibn Malik (رضئ اللہ تعالی عنہ) and placed it on his face saying: "This is the hand that touched the hand of the Messenger of Allah (صلى الله عليه و آله وسلم)"? Muslims are not moved to these matters except by their excessive love for the Prophet (صلى الله عليه و آله وسلم), as they are ordered to love Allah (سبحانہ و تعا لی) and the Prophet (صلى الله عليه و آله وسلم) more than their own lives, their children, all human beings, their property, and Paradise and its maidens. There are even some believers that love Abu Bakr (رضئ اللہ تعالی عنہ) and 'Umar (رضئ اللہ تعالی عنہ) more than themselves. [al-Dhahabi in Mu'jam al Shuyukh (1/45)] 

Hence it now stands proven that Kissing the grave of Prophet (Peace be upon him) and Saliheen is a valid opinion according to scholars. We accept that  scholars disapproved it too, but none of the parties could be mocked at. 

Putting face on grave of Prophet (Peace be upon him) or kissing it is approved from this authentic narration. 

عن داود بن أبي صالح ، قال: أقبل مروان يوماً فوجد رجلاً واضعاً وجهه على القبر فأخذ برقبته، وقال: أتدري ما تصنع؟ قال: نعم، فأقبل عليه فإذا هو أبو أيوب الأنصاري رضي الله عنه فقال: جئت رسول الله ولم آتِ الحجر، سمعت رسول الله يقول: «لا تَبْكوا علَى الدّينِ إِذا وَلِيَهُ أَهْلُهُ وَلكِنِ ابْكوا عَلَيْهِ إِذا وَلِيَهُ غَيْرُ أَهْلِهِ». 

هذا حديث صحيح الإسناد 

Translation: Dawud ibn Salih said: "[The governor of Madina] Marwan one day saw a man placing his face on top of the grave of the Prophet. He said: "Do you know what you are doing?" When he came near him, he realized it was (Sahabi) Abu Ayyub al-Ansari. The latter said: "Yes; I came to the Prophet, not to a stone." [Musnad Ahmed Bin Hanbal 5:466, Imam Hakim in his Al-Mustadrak 4:520, Hadith # 8571, Where he declared the chain as “SAHIH” and Imam Dahahbi also called it Sahih]