Microsoft Support - Windows NT 4.0 Service Pack 6a Available
MS Windows NT 4.0 Service Pack 6a (SP6a)
Winlogon Service Update for Windows NT 4.0 Service Pack 6 (SP6 & SP6a)
Microsoft Support - Post-Windows NT 4.0 Service Pack 6a Security Rollup Package (SRP)
Microsoft Support - List of bugs fixed in Windows NT 4.0 Service Pack 6/6a (Part 1)
MSFN - Windows NT 4.0 Service Pack 6a contains a list of direct links to NT 4.0 Service Pack 5 and 6 downloads on the Microsoft.com site.
Windows NT 4.0 Reference Material
Windows NT 4.0 Service Pack and Hotfixes Reference