The 4 deputies of Imam Mahdi, an authority upon Shias?

Excerpt from Ayatullah Sistani's advanced Fiqh (Islamic jurisprudence) lecture on the conditions required for a place to be considered suitable for performing salah (Islamic ritual prayer), transcribed by Allama Murtada al Mohri (Kuwaiti Shia scholar)* and published (after review and correction by Ayatullah Sistani) on 02-April 2012 by the title of "Kitabus Salah, Makanul Musalli".

*Translator's note: Allama Murtada al Mohri attended bahth al kharij lectures of Ayatullah Sistani during his tenure at the hawzat of Najaf. Later, he also served as an expert panelist at Ayatullah Sistani's office in Qum, where he would issue answers for the religious queries addressed to Ayatullah Sistani by his muqallids (followers).

 كتاب الصلاة - مكان المصلي ص ١٢٥

 "Kitabus Salah, Makanul Musalli", Page 125

فان قلت: يكفي في الحجية كونه فتوى حسين بن روح رضي اللّه عنه فانه النائب الخاص.

قلت: ليس كذلك وليس هو معصوماً ولا حجية في فتواه وهو الذي اشرف على تأليف كتاب التكليف للشلمغاني وصححه واراد بذلك ان يكون الكتاب الرسمي للشيعة واعترف بصحته علماء بغداد ولم يناقش فيه علماء قم الا في موردين واليه يرجع اكثر الاجماعات المدعاة. ومع ذلك فقد خالفه في عصره بعض العلماء وافتوا برأيهم كالكليني قدس سره

So if it is argued that even if the fatwa under discussion is from Hussain b. Roh al Nawbakhti himself individually, it is still authoritative because he was among the special four deputies of Imam al Mahdi during the minor occultation, then my response is that it is not like that. Hussain b. Roh al Nawbakhti was not an infallible therefore his individual fatwa has absolutely no authority. 

He is the one who endorsed and reviewed Shamalghani's "Kitab al Taklif", with the intention of making it an official religious guide for the Shias. The scholars of Baghdad acknowledged the accuracy of the book and the scholars of Qum also did not find any faults with it except on two issues, and many of the Imami jurisprudential consensuses have their roots in that book. However, despite that, some of the scholars in that period, such as Shaikh Kulaini ignored that book and issued religious edicts independent of it, by their own judgement (based upon ahadith).