Four-jaw chucks have four advantages over three-jaw chucks.
(1) They can be used to hold irregularly shaped parts.
(2) With the use of a dial indicator, they can be used to center parts with a great deal of accuracy.
(3) They can be used to deliberately hold a part off-center.
(4) They can clamp stock tighter.
This is a valuable asset when machining cams, crankshafts, and similar parts.
The main disadvantage of the 4-jaw independent chuck is that the jaws must be individually set, adding considerable time to setups.

Like the Sherline 3-jaw chucks, the 4-jaw chucks can be used to clamp either externally or internally.
The 2.5" (63mm) 4-jaw chuck (P/N 1044) is designed to grip from 3/32" (2mm) up to 1-3/16" (30mm) diameter stock with the jaws in the normal position.
For larger diameters, the jaws must be reversed, and can grip up to 2-1/4" (56mm). The chuck has a .687" (17mm) through hole with a 3/4-16 thread.
(The 2.5" chuck is also available with 1/2-20, 12mm x 1mm and 14mm x 1mm spindle threads to fit other brand machines. See drop-down menu below to order.)
A hex key (provided) is used to tighten each jaw individually.
Tommy bars (also provided) are used in the holes in the side to tighten and loosen the chuck for more leverage.
A groove is provided around the base of the chuck so that angle clamps can be used to mount it to the table of a mill as a holding fixture.