I am new to model builder and have not been able to figure how to use part of the FC name: For example the first FC in my GDB containing around 100 files is "A1005_GPS_Residence", and I need to calculate the new field SID = "A1005". I have seen some posts with python and manually using the Field Calculator. I tried using one expression, shown in the screenshot, but Im not sure if I can combine field calculations with inline substitution or the like !%Name%![1:4]. Thanks for any info/help.
I have attached a python script that should steer you in the right direction. You will find that custom naming in loops is much easier in python than model builder.
EDIT: Added additional code to address new information