Be sure to read about significant changes (although this is not exhaustive), and the revised Technical Overview and to a lesser extent the Environment Overview.
With the move from from HFRD to WASD the name and location of the document in [DOC.HFRDHYP] changed to [DOC.ENV] (apologies for any broken bookmarks!)
Due to changes in connection request processing some NETLIB supporting TCP/IP packages can no longer provide DNS lookup (it now occurs at AST level, see the NETLIB documentation).
This version has a separate, configurable message database capable of supporting multiple, concurrent languages. A logical name, HTTPD$MSG, needs to be provided to locate it for the server. A default file may be copied from HT_ROOT:[EXAMPLE]HTTPD$MSG.CONF to wherever the site places configuration files.
The configuration directives "ErrorInfo", "ErrorSysAdmin" and "AuthVMS" have been retired, and the following introduced, "DirDescription", "DirNoPrivIgnore", "ErrorSourceInfo", "LogFormat" and "Service".
SYSUAF-authentication (previously "AuthVMS") is now controlled using the server qualifier /SYSUAF. The qualifiers /PROFILE and /SERVICE have been introduced.
Minor changes to configuration (including mapping for the echo and Xray facilities) and startup files can be checked in the HT_ROOT:[EXAMPLE] directory.
There are some significant changes in configuration file format and server startup (as well as functional and performance improvements for that price :^)
If at all possible, restoring the new version 4 tree, reviewing the new configuration and mapping files, etc., adjusting startups, and then migrating any local applications into it, might be the cleanest approach.