If you ever need to separate part of your incoming information in a lead, such as an area code or the last four of a social security number, then sub-string is the way to do so.
Under the Client tab, choose the client you need to pull the information from. Open that client and go to their delivery method. Once inside the desired delivery method, got to the Field Mappings tab.
For the sub-string mapping, you would create a new field mapping and use the Field Type 'Sub-String'.
The above example will pass into a field called BirthYear the value captured from taking the first 4 digits from the BirthDate field.
If you think of your value like this
1 9 8 4 / 0 7 / 0 1
0 1 2 3 4 5 6 7 8 9
You are starting in the 0 placeholder and taking 4 digits. This is what the Expression value [BirthDate],0,4 means in the example above.
The resulting value would be 1984.