I'd just use s/^(\d{4})$/$1_001/

~>perl -e '@id = (1234,"4323_003"); foreach ( @id ) { s/^(\d{4})$/$1_0
+01/; print $_, "\n"; }'
1234_001
4323_003
[download]

If the input needs to be checked, I'd do it separately (and before).

qq