Although there weren't that many differences between IIS 4 and IIS 5, a few items warrant a mention, both in terms of their difference from IIS 4 to IIS 5 and on to IIS 6:
A slight name change? IIS was Internet Information Server; it became Internet Information Services. The name change reflects the change from a basic Web server to a more generic Internet server.
Internet Services Manager? This has now become IIS Manager and is part of the Administrative Tools command set.
Active Directory? Again introduced with IIS 5, it's integration has been further expanded with IIS 6.
CPU throttling? Introduced in IIS 5 (and as demonstrated in Figure 7.2) this has now been expanded upon. Instead of a single performance tuning element within the Performance tab, it's the worker processes that are used to define the performance characteristics of a given application or site.
WebDAV? Support for Web Distributed Authoring and Versioning, which allows clients and applications to edit and update information on a Web site directly and much more easily. It's also a better alternative than using FrontPage.