Postgres 9.6

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create table calendar
(
    id serial primary key, 
    cal_day date not null,
    business_day bool not null
);

CREATE TABLE

insert into calendar (cal_day, business_day) values 
('20180101', false), ('20180102', true),
('20180103', false), ('20180104', true),
('20180105', false), ('20180106', true),
('20180107', false), ('20180108', true),
('20180109', false), ('20180110', true),
('20180111', false), ('20180112', true);

INSERT 0 12

select max(cal_day) as the_day
from (
      select   cal_day
      from     calendar
      where    cal_day > '20180103'
      and      business_day = true
      order by cal_day
      limit 4
     ) bd;

the_day

2018-01-10

SELECT 1

select cal_day
from   (select cal_day,
               row_number() over (order by cal_day) rn
        from   calendar
        where  cal_day > '20180103'
        and    business_day = true
        limit  4) bd
where  rn = 4;

cal_day

2018-01-10

SELECT 1

create or replace function add_business_day(from_date date, num_days int)
returns date
as $fbd$
    
    select max(cal_day) as the_day
    from (select   cal_day
          from     calendar
          where    cal_day > $1
          and      business_day = true
          order by cal_day
          limit    $2) bd;

$fbd$ language sql;

CREATE FUNCTION

create or replace function add_business_day2(from_date date, num_days int)
returns date
as $fbd$
    
    select cal_day
    from   (select cal_day,
                   row_number() over (order by cal_day) rn
            from   calendar
            where  cal_day > $1
            and    business_day = true
            limit  $2) bd
    where  rn = $2;

$fbd$ language sql;

CREATE FUNCTION

select add_business_day('20180103', 4);

add_business_day

2018-01-10

SELECT 1

select add_business_day2('20180103', 4)

add_business_day2

2018-01-10

SELECT 1