On a horse-drawn vehicle: a pivoting bar or set of bars used to equalize the force from four singletrees.
1925 Frederick Post (MD) 21 Feb 4/5, [Advt:] Fourble, triple, double and single trees. 1949 PADS 11.6 wTX (as of c1920), Fourble-tree. . . A pivoted bar to which four whiffletrees are attached. 1956 Ker Vocab. W. TX 179, For a bar to which three or four whippletrees are attached five informants report tripletree, trebbletree, tribletree, fobletree, fourbletree and evener. 1962 Atwood Vocab. TX 52, A few informants volunteered terms for a longer bar [than a doubletree] in case there were three or more horses; these are tripletree . . , trebletree . . , thribbletree . . , and fourbletree. 1966 San Antonio Light (TX) 26 Oct 67/1, A fourble tree is what you use when you are working four animals abreast, whether it’s in plowing or in drawing a heavy load of anything. I have known this ever since I was a little girl living on a farm near Henrietta, in Clay County. Ibid 3 Dec 8/1 MN (as of c1916), I used to hear both the terms ‘fourble tree’ and ‘evener’ used when I was young and working on a farm in southern Minnesota about 50 years ago.