R 4 questions.

Westfork, I am not sure but I think all of the clutch linkage and most of the other running gear parts would be the same as the RD4, which was produced in much greater number, therefore more used parts are available. More experienced people might chime in and confirm or deny this. Good luck Grant.

Hi Don,
pages 132 to 134 in the following link from General Gear/Tractor Parts available Cat Parts, Service and Operation Manuals,
shows the style of linkage used in the R4, and D4's from earliest up to U Series, also including the later Thirty.

http://www.tractorparts.com/PDFs/CAT_D4-4GPARTSlargepps.pdf

Link to page with said manuals, go to the bottom of the page to find them.

http://www.tpaktopc.net/d4strcltch.htm

Could be failed throw out bearing, broken bell crank actuation ear on throw out bearing cage, broken bell crank lever or even the bell crank lever shaft broken at one of the bell crank lever retainer clevis pin holes.
As you have seen the linkage you may be able to ascertain the failed parts from above descriptions that i have seen all off over the years.
From this you can see that these do not use the conventional throw out yoke pivoting in the lower of the case.

To get at this area you need to remove the transmission cover to remove the rivets retaining the bell crank lever shaft in the main case.
There should be corks in tapped holes in the outer ends of these shafts to facilitate removing the shafts with a puller bolt/all thread & nut and spacers from/thru the rear face of the case.
Sorry, have run out of attachment space to post pix etc. so have done it this way.
Cheers,
Eddie B.