@AmigaBlitter
It depends on what you need. Expat is fast and available for OS4 in all forms (shared Amiga library, static lib, shared object) but it only provides an event-driven parser for which you need to write your own callback functions to handle the individual types of XML event. Plus, if you need a library that builds a DOM-tree representation of your XML file, which you can then manipulate, Expat will not help you.
In such a case you pretty much only have two options: libxml2 and
libroxml (which you'll have to port yourself but last time I tried the porting was easy). libxml2 is much more heavyweight and memory-hungry but it's a cross-platform standard that has received a lot of testing and feedback over the years. Further, if you also need XSLT support then libxml2 is a natural choice because it twins with libxslt (developed by the same person). The latest version of libxml2 (2.9.7) has already been ported by MickJT and libxslt is in the pipeline (hasn't made it to OS4depot yet).
Hope this helps.