Just set up a new Mac, and after installing the latest Fork (2.29.2) and the latest Kaleidoscope (Version 4.0.3 (3824)) and using its Integrations view to install ksdiff I nevertheless cannot select Kaleidoscope as my Merge Tool or External Diff Tool in Fork.
In the menus in the Integration pane of Fork's settings window, there are Kaleidoscope2 and Kaleidoscope3 menu items, but they are disabled.
Perhaps it needs an update for Kaleidoscope 4?