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If the first residue in the structure file for a starting PDB is 4, to change this to 0, write in PyMol:
alter name, resi=str(int(resi)-4) (name= object name or sele) → subtract 4 from each residue number; + number would add)
With the same command, you also can rename a chain. Select group of residues and then type
alter sele, chain='A' (chain A is defined)