If u look here at Imam Ibn Hajr’s treatment of the topic, there is a wealth of info. He basically says there have predominantly been two types of opinion on the issue:
1) Not cutting it at all.
2) Cutting what is extra, but no one says less than a fistfull.
I will try to summarize:
1) Al-Kirmani [whom ibn hajr isnt very fond of] , one of the exegetes on Bukhari said, it maybe out of ijtihaad of ibn umar that he tried to act upon both shortening the hair and shaving them after umrah and hajj, as he shaved his head and trimmed his beard. This is ellucidated in a fuller version of the report that when ibn ‘umar would shave hs head for hajj and umrah he would also trim the mustache and the beard. This narration is in Muwatta of Imam Malik. Al-Kirmani said that he did this to gather both the commandments a) of trimming and b) of shaving in one act and in times other than hajj and umrah he acted upon the generality of “leave the beards” [i.e the hadith in this regard narrated by ibn ‘umar]
2) Ibn Hajr rejects this and says that there is proof to the contrary that ibn ‘umar did not restrict that to hajj and ‘umrah, however in my very humble opinion, that is not correct since it is not clear from the narrations he presents (below in point 3) that it was in other than hajj and ‘umrah.
3) Al-Tabari presented three opinions in the scholars:
a) A group which dislikes trimming anything of the beard at anytime. They take the apparent/obvious (dhaahir) meaning of the ahadith.
b) A group who says that whenever it exceeds a fist, it can be trimmed. Al-Tabari said that ibn ‘umar did that and that he trimmed the beard of another man [the athar of Ibn ‘umar is about hajj and ‘umrah, not general as tabari and ibn hajr claimed, rahimahumallah]. And that Abou Hurairah did so. Also abu dawood with a hasan chain narrated from Jabir b ‘Adbullah that “We used to leave the long hair of the beard, except in Hajj and ‘umrah”.
c) Some scholars say that it should not be trimmed less than a fist. Al-Hasan Al-Basri and ‘Ataa b Abi Rabaah [shaykh of imam abi hanifa] said it is ok to trim only so much so that it does not becomes ridiculously big, in which case he would be making himself liable to ridicule by others. Ibn Al-Teen states that whatever is below the fist should be removes (not just the long hair) to make the length of the beard’s hair at one level.
d) A group which disliked to trim it at all, except in hajj and ‘umrah. This is the saying of Al-Nawawi too. Imam Shafa’ii feels it recommended in hajj and ‘umrah

In conlusion, in my humble opinion, i think if we lookat all the athaar of the sahabah, the different ahadith, the fact that the Prophet sallallaho’alaihiwasallam never did that or was never reported to do so,
1) it is impermissble to trim the beard in general times.
2) It is ok to do so in hajj and ‘umrah, though not i believe to just four fingers long, the fist, but to make a fist from the bottom to trim a little , as a middle ground between the conflicting opinions. It is highly improbable that the sahabah would invent it themselves or so many of them would do it out of ijtihaad, where ijtihaad is impermissible, i.e in worships. As Jabir said, they would leave the extra long hairs, untill the hajj and ‘umrah.
3) Cutting less than a fist is neither supported from sahabah nor the salaf.

Allahu A’alam.