You need to supply a format mask. In PostgreSQL there is no default:
select to_char(1234, 'FM9999');
If you don’t know how many digits there are, just estimate the maximum:
select to_char(1234, 'FM999999999999999999');
If the number has less digits, this won’t have any side effects.
If you don’t need any formatting (like decimal point, thousands separator) you can also simply cast the value to text:
select 1234::text