The H3 library introduced breaking changes in version 4.0.0, so applications developed against earlier versions need to be updated before they can use the new library version. Most of the changes relate to function naming, and some relate to the behavior of the functions in some cases.
The grid itself - the layout of cells, which cell an H3 index refers to, and the structure of an H3 index - is not changed in 4.0.0. H3 indexes generated by 3.x will be understood the same by 4.0.0, and those generated by 4.0.0 will be understood the same in 3.x.
The areas where there are changes:
Many function names have changed between 3.x and 4.0.0. These follow the H3 terminology reference, and that document explains how each function name has been renamed. These changes were made to make the API more consistent and predictable.
The C library is being changed to more consistently report errors. This is done via the return code of functions. Users of the C library will need to adjust some function calls to pass output parameters instead of using the return value of a function.