Thanks to John and John Nuechterlein for the tip: Microsoft has released three new post-SP4 hot-fixes this week, including:
GetHostByName API fixes (rnr-fix)
Windows NT 4.0 Service Pack 4 causes NDISWAN Services to return IP addresses in the order in which they are bound. This change results in IP addresses not currently bound to the TCP/IP stack being returned when making a call to the Winsock API function GetHostByName. This causes problems with Exchange Server as well as NT itself.
WinNT 4.0 Post-Service Pack 4 Hot-fixes Combined Into One Package
A combination of previous post-SP4 hot-fixes in one convenient install package.
RRAS Computer Stops Responding to Incoming Calls Under Stress
The Routing and Remote Access Server (RRAS) computer may stop responding to incoming calls after a while.