As you mention, the What's New in SAS 9.4 doc is the definitive location for a list of new features/functionality in 9.4M4. Since this guide contains information on all 9.4 maintenance releases, you'll need to search for references to the "fourth maintenance release" to get a list specifically tailored for this latest maintenance. That said, here's a quick, but certainly not exhaustive, list of associate enhancements:
JSON LIBNAME engine
ODS accessibility support (tagged PDF output)
Updates to certain included 3rd party software
SAS Deployment Wizard enhancements, especially related to configuration updating
SAS Studio 3.6
Enterprise Guide 7.13, including enhanced DATA step debugger
New 14.2 versions of Advanced Analytics products (SAS/STAT, SAS/ETS, SAS/IML, ...)
SAS Viya enablement of code node in SAS Enterprise Miner, SAS Studio, and DI Studio
I encourage you to consult the What's New guide for a more complete list.
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