If the field is a number field it could be done with series of selects using less than and greater than. If the field is text you could use the "like" parameter.
For a text field: Select LocID like '0000*' and calculate those. Then Select LocID like '000*' and calculate those.
For a number field: Select where n > 99 and n < 1000. That gets all of the 3 digit numbers and then calculate new field. Select where n >999 and n< 10000. That gets all the 4 digit numbers and then calculate new field.
That syntax isn't exactly right, but you get the idea. I know it's a bit clunky, but it will get you there.