A rough draft using some dummy data:

select 
    json_build_object('promotions', jsonb_build_object('one', 1, 'two', 2), 'items', ARRAY[1, 2]) AS json_test;

json_test                        
--------------------------------------------------------
 {"promotions" : {"one": 1, "two": 2}, "items" : [1,2]}

This should serve as starting point.

Postgres 10+

Convert the whole row to a jsonb object and eliminate a single key (pg 9.5+) or an array of keys (pg 10+) with the - operator before aggregating:

SELECT val2, jsonb_agg(to_jsonb(t.*) - '{id, val2}'::text[]) AS js_34
FROM   tbl t
GROUP  BY val2;

The explicit cast in '{id, val2}'::text[] is necessary to disambiguate from the overloaded function taking a single key as text.

See:

• Select columns inside jsonb_agg

Postgres 9.4+

jsonb_build_object() or json_build_object().

SELECT val2, jsonb_agg(jsonb_build_object('val3', val3, 'val4', val4)) AS js_34
FROM   tbl 
GROUP  BY val2;

The manual:

Builds a JSON object out of a variadic argument list. By convention, the argument list consists of alternating keys and values.

Postgres 9.3+

to_jsonb() (or to_json) with a ROW expression does the trick. (Or row_to_json() with optional line feeds):

SELECT val2, jsonb_agg(to_jsonb((val3, val4))) AS js_34
FROM   tbl
GROUP  BY val2;

But you lose original column names. A cast to a registered row type avoids that. (The row type of a temporary table serves for ad hoc queries, too.)

CREATE TYPE foo AS (val3 text, val4 text);  -- once in the same session

SELECT val2, jsonb_agg((val3, val4)::foo) AS js_34
FROM   tbl
GROUP  BY val2;

Or use a subselect instead of the ROW expression. More verbose, but without type cast:

SELECT val2, jsonb_agg(to_jsonb((SELECT t FROM (SELECT val3, val4) t))) AS js_34
FROM   tbl
GROUP  BY val2;

to_jsonb() is an optional addition to add (insignificant) line breaks in the JSON document.

More in Craig's related answer:

• Using row_to_json() with nested joins

fiddle
Old sqlfiddle