File "%AppData%\Processing\preferences.txt" is where Processing looks up its configuration.
Among them we have properties such as these 3:
sketchbook.path=sketchbook.path.three=sketchbook.path.four=
Which determines the each “Sketchbook” path for Processing versions 2, 3 & 4 respectively.
Again, have utmost care when upgrading Processing to a major version, as not to mix those paths!
Otherwise libraries, modes and tools will conflict w/ each other due to major version incompatibility!