One of the vital witnesses used to support the presence of the phrase 'let us go into the field' in Genesis 4:8 is in fact a witness against its inclusion ...
In the deadly fraternal encounter of Cain and Abel in Genesis 4 there is a well known textual dispute. The Masoretic Text is most naturally translated thus:
'And Cain said to Abel his brother. And it happened, when they were in the field, Cain arose against Abel his brother and killed him.'
The problem in MT is the appearance of the phrase 'and he said' or ויאמר without any quoted speech. Westermann (Genesis 1-11: A Commentary [London: SPCK, 1984], p. 302) is typical of commentators in asserting that the Samaritan Pentateuch, LXX, Peshitta, and Vulgate seem to supply נלכה השדה 'let us go into the field'.
If we ignore proposals of emendation, commentators usually choose one of the following options:
1) MT has a lacuna that has been creatively supplied by versions (not necessarily independently of each other);
2) The versions preserve words which dropped out of MT;
3) MT can be retained by supposing that ויאמר 'and he said' does not have to be followed by direct speech, being equivalent to 'and he spoke' (thus, for example, the KJV).
Undoubtedly both readings—the one including the words נלכה השדה and the one excluding them—have a high antiquity. MT omits the words, but some manuscripts show a space (pisqah be'emsa' pasuq) that may show knowledge of a tradition including the words. Furthermore, 4QGen-b from the Dead Sea Scrolls, a manuscript remarkable because throughout it only differs from the consonants of MT in one minor point of spelling, lacks the words. The Samaritan Pentateuch is the only Hebrew witness for the inclusion of the words. The reading of the LXX, διεθωμεν εις το πεδιον, is best explained as a translation of נלכה השדה since in 85 out of 129 occurrences of πεδιον or its plural it corresponds to שדה or its plural in MT.
It is frequently added at this point that the Peshitta also supports the longer reading. U. Cassuto, for example, claims that the Peshitta reads 'let us go into the open country' (A Commentary on the Book of Genesis, Part 1 [Jerusalem: Magnes, 1961], p. 214). The editors of 4QGen-b agree that the Peshitta supports SP ([DJD XII; Oxford: Clarendon, 1994], p. 37). However, the Syriac in fact reads 'let us travel to the valley' (ܢܪܕܐ ܠܦܩܥܬܐ [nrd' lpq't']). It is precisely at this point that we see that the Syriac has not arisen from a Hebrew Vorlage, but from consultation of the Greek. Weitzman holds that the Syriac ܦܩܥܬܐ (pq't') may have meant 'plain' originally, and seeks to support this on the basis of LXX's πεδιον (M.P. Weitzman, The Syriac Version of the Old Testament [Cambridge: CUP, 1999], pp. 74-75). However, it is possible to look at this from another angle. Though the LXX's πεδιον may mean 'plain', the Syriac translator (having no access to the Hebrew נלכה השדה) has understood it as 'valley'. The Peshitta seems generally to have been translated from a Hebrew text, but that the translators occasionally consulted the LXX, especially in places of difficulty. It is consistent with this if we suppose that here the Peshitta is in fact a witness to the absence of נלכה השדה in its Vorlage. If it had השדה why would it not translate with ܚܩܠܐ the normal word for 'field' as in fact it did for שדה later in Genesis 4:8? The best explanation for the difference of word in the Syriac is that for the first occurrence the translators did not have a Hebrew text, but for the second they did.
It is suprising that Étan Levine, in an important treatment of this passage in Syriac, consistently renders the Peshitta's phrase as 'let us descend into the valley'. The mistranslation 'descend' seems to arise from confusing the Syriac word ܪܕܐ (rd') 'travel' with Hebrew ירד 'descend'. See E. Levine, 'The Syriac Version of Genesis IV 1-16', Vetus Testamentum 26 (1976) 70-78, esp. 71-72.