Daniel Morris asks if the No. 3 and No. 4 roller assemblies are interchangeable. I am answering in a new post so that I may add images –PM
The top frame assembly (one large diameter steel oscillating roller and two small diameter steel riders) is identical (and removable). The difference is in the bottom frame assembly (rubber form rollers), which in the No. 3 is attached via a bracket to the lift arm. Attached to the lift arm is a rubber feeder roller with a hand wheel that helps distribute ink. It may be possible to detach the bracket to use a No. 3 bottom frame on a No. 4 and vise versa. Let us know, Daniel! See photos and drawings below.
The photo below is another view of the same press as above (with both form roller gears on far side of press as configured on a No. 4). More importantly, it shows that these gears use the same rack as the cylinder gear. Original photos by Steve Miller.