There are not that major non-backwards-compatible changes between libyang version 3 and 4 and the version was increased in part due to new functionality. The main new feature, which most of the changes are related to, is the ability to serialize (print) compiled YANG context. What is more, such a printed context does not have to be deserialized to be used by applications and they can directly read the printed context in run-time. This saves time when reusing contexts since they do not always have to be created in every process but also potentially a lot of memory since all the processes on the system may share a single printed context. Naturally, such a concept has some limitations that are mentioned in detail in the documentation (Printed Context).

Related to this feature there were lots of minor changes to make sure all the intended functionality works. Most importantly, there are now 2 dictionaries for a context. One, read-only for the strings in the compiled context and the other for YANG data that can be written to normally. The main consequence of this is that comparing directly (the addresses) of strings from these 2 dictionaries will not work. Next, all plugin references were replaced by an uintptr_t value but there are functions lysc_get_type_plugin() and lysc_get_ext_plugin() for getting the actual plugin structures. Finally, some new callbacks were required for extension-instance plugins so that they can be printed with the context as well.

Other notable changes include major LYB binary data format optimizations. The effect in API is that wherever a size of a value is required, it is now expected in bits instead of bytes. Next, there are extension plugin API improvements to account for custom XPath root definitions. Then, obsolete nodes are not compiled and cannot be instantiated in data by default but can be if requested by a context flag. Finally, the "default" metadata instance connected to the ietf-netconf-with-defaults YANG module is now in the proper namespace, which is not the namespace of this module. Other minor NBC changes should be reported by the compilator and they should be explained in the documentation of the corresponding changed functions.

The complete generated list of changes (without any additional notes) can be found in the compatibility report.