How to fill some column with constant string followed by variable number

I need to fill a table column (in Oracle database) with string values that have variable part, e. g. AB0001, AB0002,...,AB0112...,AB9999, where AB is constant string part, 0001 -9999 is variable number part. i've tried the following solution in SQL for a table with 2 columns:

    create table tbl1
    (seq1 number(8),
     string1 varchar(32));

    declare
    tst number(8) :=0;
    begin
      for cntr in 1..100
      loop
        tst := cntr;
        insert into TBL1 values (someseq.nextval, concat('AB',tst));
      end loop;
    end;

But in this case I get STRING1 filled with values AB1,AB2,...,AB10,.. which is not exactly what I need. How should I modify my script to insert values like AB0001,...,AB0010?