sebpa=# insert into numbers (i) select random() * 5 from generate_series(1,7);
INSERT 0 7
sebpa=# select * from numbers;
  id  | i
------+---
 4001 | 0
 4002 | 1
 4003 | 4
 4004 | 1
 4005 | 3
 4006 | 3
 4007 | 1
(7 rows)

sebpa=# select id, i, sum(i) over( order by id rows between 0 preceding and 4 following) from numbers;
  id  | i | sum
------+---+-----
 4001 | 0 |   9
 4002 | 1 |  12
 4003 | 4 |  12
 4004 | 1 |   8
 4005 | 3 |   7
 4006 | 3 |   4
 4007 | 1 |   1
(7 rows)