ORDER BY ordinal position

For this query:

select first_name, hire_date, salary, manager_id mid from employees
where department_id in(110,100)
order by 4,2,3

My question is: what does the last line "order by 4,2,3" mean?

Those numbers refer to the ordinal position of the columns within the result set that determine the sequence of the result set. The ORDER BY sequence is:

the 4th column, i.e. manager_id

the 2nd column, i.e. hire_date

the 3rd column, i.e. salary

Each of these will be in ascending sequence, since that is the default.
Admittedly, it's easier to type order by 4,2,3 than it is to type order by manager_id, hire_date, salary. However, since this is the age of cut 'n' paste, it
shouldn't be too hard to construct the ORDER BY clause by pasting column names from the SELECT clause.
The reason you want to spell out, by name, which columns to sort by, is to make the query more understandable to someone not familiar with what the query is doing. Remember, this someone will
be you, six months down the road, after you have forgotten why it was 4,2,3 and not 2,4,3. If the query uses ordinal position numbers, you are forced to read the SELECT clause and count columns, which can be daunting if they include expressions as well as
simple column names.