Service Pack 2 for Windows NT 4.0 allows you to add the following enhancement to your password policies:

1. Passwords must be a minimum of 6 characters in length.
2. Password must include at least 3 of the following:
    a. English upper case letters
    b. English lower case letters
    c. 0 through 9
    d. special characters

To implement these additional requirements, copy Passfilt.dll to your%windir%\System32 directory (you may have to expand the service pack into a temp directory to locate it).

Edit: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa

Double-click on the Notification Packages value (or add it as a REG_MULTI_SZ)
Insert PASSFILT (beneath the FPNWCLNT if it is present).

You must reboot for this change to take effect.