Q:

Compute running sum in a window function

I have issues with this running sum in Redshift (uses Postgres 8):
select extract(month from registration_time) as month
 , extract(week from registration_time)%4+1 as week
 , extract(day from registration_time) as day
 , count(*) as count_of_users_registered
 , sum(count(*)) over (ORDER BY (1,2,3))
from loyalty.v_user
group by 1,2,3
order by 1,2,3
;

The error I get is:

ERROR: 42601: Aggregate window functions with an ORDER BY clause require a frame clause

A:

You can run a window functions on the result of aggregate function on the same query level. It's just much simpler to use a subquery in this case:
SELECT *, sum(count_registered_users) OVER (ORDER BY month, week, day) AS running_sum
FROM  (
   SELECT extract(month FROM registration_time)::int     AS month
        , extract(week  FROM registration_time)::int%4+1 AS week
        , extract(day   FROM registration_time)::int     AS day
        , count(*) AS count_registered_users
   FROM   loyalty.v_user
   GROUP  BY 1, 2, 3
   ORDER  BY 1, 2, 3
   ) sub;

I also fixed the syntax for expression computing week. extract() returns double precision, but the modulo operator % does not accept double precision numbers. I cast all three to integer while being at it.
Like @a_horse commented, you cannot use positional references in the ORDER BY clause of a window function (unlike in the ORDER BY clause of the query).
However, you cannot use over (order by registration_time) either in this query, since you are grouping by month, week, day. registration_time is neither aggregated nor in the GROUP BY clause as would be required. At that stage of the query evaluation, you cannot access the column any more.
You could repeat the expressions of the first three SELECT items in the ORDER BY clause to make it work:
SELECT extract(month FROM registration_time)::int     AS month
     , extract(week  FROM registration_time)::int%4+1 AS week
     , extract(day   FROM registration_time)::int     AS day
     , count(*) AS count_registered_users
     , sum(count(*)) OVER (ORDER BY 
              extract(month FROM registration_time)::int
            , extract(week  FROM registration_time)::int%4+1
            , extract(day   FROM registration_time)::int) AS running_sum
FROM   loyalty.v_user
GROUP  BY 1, 2, 3
ORDER  BY 1, 2, 3;

But that seems rather noisy. (Performance would be good, though.)
Aside: I do wonder about the purpose behind week%4+1 ... The whole query might be simpler.
Related:

Get the distinct sum of a joined table column
PostgreSQL: running count of rows for a query 'by minute'

