I know that there are only four Sunni schools of Fiqh. Namely Shafiite, Hanbali, Maliki & Hanafi. And all followers do their Islamic duties according to only four schools. Is it possible to do ijtihad not according to these four schools?
"Is it possible to do ijtihad not according to these four schools?"
yes. see meaning of the word : https://en.wikipedia.org/wiki/Ijtihad :
Ijtihad (Arabic: اجتهاد ijtihād, "diligence") is an Islamic legal term that means "independent reasoning" or "the utmost effort an individual can put forth in an activity."
yes, of course, independent reasoning is possible. i do not know anything in quran nor hadithes that prohibit that.
it is not only possible, it is normal and must be used. because independendent reasoning is, in another words, just thinking about how to do things islamically and with more sawab. of course, it is not prohibited and it is accounted a good thing. (see ayats with word of "think": http://corpus.quran.com/qurandictionary.jsp?q=fkr ; http://corpus.quran.com/qurandictionary.jsp?q=Eql ; "proof" : http://corpus.quran.com/qurandictionary.jsp?q=brhn ).
who follow other persons, if he does wrong, will get punishment: http://quran.com/6/164 :
... And every soul earns not [blame] except against itself, and no bearer of burdens will bear the burden of another. ...
They will say, "Nor you! No welcome for you. You, [our leaders], brought this upon us, and wretched is the settlement." They will say, "Our Lord, whoever brought this upon us - increase for him double punishment in the Fire."
so, in some cases, if you see that reasons of fakihs are incorrect, you should better think by yourself.
persons who disallow ijtihad use this ayat: http://quran.com/16/43 :
And We sent not before you except men to whom We revealed [Our message]. So ask the people of the message if you do not know.
but it does not say that you always must ask and follow fakihs, alims, who know that quran, hadithes, etc by heart, (as the disallowers say that they must be such persons). words/phrase used there are "ahlu z zikr", ahlu is people or like people ( http://corpus.quran.com/qurandictionary.jsp?q=Ahl ) and zikr is remembering, reminding : http://corpus.quran.com/qurandictionary.jsp?q=*kr - i think it is because people of time of muhammad used to rememeber lots of texts, and they had to read that texts time to time , to not forget them. and nowadays it is easy to search texts in internet, though knowing men also can be very helpful, because they get from their memory from many fields of knowledge, and have/know connections between that fields.
and this ayat does not say to ask and follow them always, but only to ask things which you do not know.
and using of ijtihad may be impossible to bypass/evade (ie may be necessary):
- if you need to do something today but you can get answer from an islamic scholar only tomorrow.
- there are lot of different acts and decisions for them, so, asking for everything scholars would bury them with questions. and that things are not all considered in mazhab scholars texts.
- sometimes, you cannot follow a mazhab, if their texts are not translated to language that you know and you do not know arabic, or if you cannot access to that texts at all, and you cannot find a person who can answer to you.
First I should say that your Questions are very difficult to answer briefly and i'm not sure that i will be good at it but i'll try to:
To answer the Question above Does ijtihad open in Islam (Sunni view)?
is very difficult as, 1st what you mean with ijtihad? If it comes to any kind of ijtihad none of the Schools would like to say/admit that the doors of ijtihad are closed and will say that it is fard! Some even say ijtihad has no door!, so how it could be closed!
Ibn A'rafa al maliki said: our Sheik ibn Abd as-Salam (a Imam of the Maliki school) said: There will be now time without a mujtahid except the time knowledge/wisdom will be taken (by Allah)
قال شيخنا ابن عبد السلام ،يعني أحد أئمة المالكية، : لا يخلو الزمان عن مجتهد إلى زمن انقطاع العلم
As-Suyuti said: all the Hanbalis agree that there shouldn't be a time without a mujtahid because the Messenger of Allah (peace be upon him) said "There will never cease to be a group from my Ummah manifest upon the truth, they will not be harmed by those who forsake them until Allah's Decree comes." (Sahih Muslim and al-Bukhari and others) because ijtihad is fard kifaya, so if there will be no ijtihad this would mean that the Muslims agree that this it would be evil
" ذهبت الحنابلة بأسرهم إلى أنه لا يجوز خلو الزمان عن مجتهد لقوله - صلى الله عليه وسلم- لا تزال طائفة من أمتي ظاهرين على الحق حتى يأتي أمر الله رواه الشيخان وغيرهما قالوا لأن الاجتهاد فرض كفاية فيستلزم انتفاؤه اتفاق المسلمين على الباطل"
Both statements are mentioned in the book of Imam Suyuti الرد على من أخلد إلى الأرض وجهل أن الاجتهاد في كل عصر فرض للسيوطي
There are many different forms and a kind ranking of ijtihad i will try to explain later.
About the sunni fiqh schools (historical retrospection)
There are many sunni school of fiqh. But there are only a few which are well documented and have a lot of more or less good explanation and interpretation of their rules, sources etc.
The 4 Schools you mentioned are the best known and best documented ones.
You should know that beside those and around the 2nd hijri Century (and from the beginning of the Khilafa) there have been many others like Imam al-Awza'y الأوزاعي, Al-Laith ibn Sa'ad الليث بن سعد (in Egypt), the 2 soufians: ath-Thawry سفيان الثوري and ibn U'yayna سفيان بن عيينة all those and many others had their fiqh school but not enough or not hardworking followers/students who kept their mdahabs alive. After that their was also Imam at-Tabari أبو جعفر محمد بن جرير الطبري and the School of az-Zahriya الظاهرية which has quasi ended with ibn Hazm ابن حزم but some of the ideas are alive in what is called Salafi madhab espacially when it comes to interpretation/exegesis of the Quran.
By the way the Zahiriya seem to me a school where even if it has declared sources the scholar is very free to do ijtihad this means that theoretically at any time a Zahiry scholar could change about the whole Opinions of the madhab according to his view/interprteation of Quran and Sunna!
But back in history one also should know that the sahaba (may Allah be pleased with them) and after them tabi'yn had their own madhabs. For example the Hanafi school was affected by the madhab of ibn Masu'd (May Allah be pleased with him) while the Maliki school was afected by the madhab of ibn O'mar (May Allah be pleased with them)
And maybe one may consider ibn Taymiyah or ibn al-Qayyim as one of the last scholars who tried to do ijtihad (i'm not in the position to evaluate), some scholars of the salafi "school" nowadays (people who are going the same way as ibn Taymiyah, ibn al-Qayyim and maybe ibn abd al-Wahhab) may (pretend to) do ijtihad (Allah knows best) But the salafi school is basically the same as the Hanbali school!
Ranking of Ijtihad
Ijtihad is defined as an Islamic legal term that means "independent reasoning" or "the utmost effort an individual can put forth in an activity. By using both the Qu'ran and Hadith as resources, the scholar is required to carefully rely on analogical reasoning to find a solution to a legal problem, which is considered to be a religious duty for those qualified to conduct it. Thus, a mujtahid is recognized as an Islamic scholar who is competent in interpreting sharia by ijtihad. Today, there are many different opinions surrounding the role of ijtihad in modern society.
- The absolut mujtahid المجتهد المطلق: one could also say an independant mujtahid like the 4 Imams of the fiqh schools and anybody who went to do ijitihad according and from the sources of islamic law!
- The mujtahid inside a madhab (school) المجتهد في المذهب: this one is doing ijtihad inside a school following its osol like abu Yusuf and Mohammed ibn al Hassan in the Hanafi madhab.
- The mujtahid of fatwa or favoring مجتهد الترجيح: This is one who could favorise between the statments of different madhabs
- The mujtahid in special terms: This one is specialised in fatwa about something special like for example sawm/prayer ...
Conclusion: Yes it is possible to do ijtihad outside of these schools, but this would mean a lot of hard work and knowing the sources very well being very good in Arabic language etc. -> see for example in wikipedia Qualifications of a mujtahid شروط المجتهد and therefore quasi nobody try it and only few did so in the last 8 century's