postgresql - postgres offset by value not number -

i have table @ least "name" column , "ordinal_position" column. wish loop each row starting row user inputs. let's user inputs "john", , ordinal_position 6 (out of 10 total). how loop last 4 rows without using subquery? i've tried using "over()" window function doesn't seem work on offset part of query, , same offset takes numbers (as far know) not strings.

edit (in response klin):

insert foo(id,name,ordinal_position) values (default,'peter',1), (default,'james',2), (default,'freddy',3), (default,'mark',4), (default,'jack',5), (default,'john',6), (default,'will',7), (default,'robert',8), (default,'dave',9), (default,'michael',10); 

so in for, since user inputed "john" want loop through will-michael. following without subquery:

select * foo order ordinal_position offset     (select ordinal_position foo name='john'); 

unfortunately, have query table find ordinal_position given name. however, not use offset. can in where clause, large tables faster:

select * foo ordinal_position > (select ordinal_position foo name = 'john') order ordinal_position;   id |  name   | ordinal_position  ----+---------+------------------   7 |    |                7   8 | robert  |                8   9 | dave    |                9  10 | michael |               10 (4 rows)