If you have a situation where you have a FEW main categories under which folders may be filed you can add a letter code after the first number in your four digit you should have a separate index book for each category. If you do this be sure to add the letter code to the file folder and all entries in the index relating to that file. All the file folders in that category should be in a separate file drawer! A word of WARNING here!
It will become very confusing if you do! At the moment because of my business and home situations I have the following categories:
0000 With no suffix - Insurance and other business records including taxes not related to specific clients. 0000S Securities records not relating to specific clients. Client files are filed separately alphabetically by name and don't need this system. 0000P Personal home information and records. 0000T Is for my video and audio tape file that is noted in the index by tape number the same as file folders are numbered. However, since more than one topic can be on a tape, in the alphabetical index they're referenced by both tape number and the counter number on my recorder to locate the part of the tape the topic is located on.
These are all the categories I use, and I have a pretty extensive file system. Again remember: DON'T CREATE A LOT OF CATEGORIES! IT GETS TOO CONFUSING! Now raise your right hand and repeat after me: