Maybe you need something like
Update CodeTbl set num = "4555"||num;
Some Sql/DB Engines need a 'where' clause - so you could do
Update CodeTbl set num = "4555"||num where num not null;
you didnt say what SQL you're using but thats the general idea ....
|| in some SQL means Concat ..the other assumption Im making, since you dont mention it, is that 'num' is actually a string as shown in your representation