Software

Scripting only works if you can logically separate the users in some way. Either by OU, group, name, description, extended attribute, etc. Once you learn what makes these 4000 users unique, then you can script adding them to a group. What might help you is to think of the users a multiple groups all part of a larger group. In that way you may be able to create AD searches to get the users in one screen, select them all, and then right-click -> add to group. Remember, your set theory works here, you are doing a union (ie. a user can only be added to the group once so don't worry if they are in more than one of your smaller groups. You will get an error adding them, but it is pretty easy to click through them in ADUC.