Select bottom 25%

I am trying to find the bottom 25% value of the 'Value' column, broken up by MainGroup ideally broken into 4 quarters.
Any help would be appreciated.

IF OBJECT_ID('TempDB..#mytable','U') IS NOT NULL
DROP TABLE #mytable

CREATE TABLE #mytable
(
MainGroup varchar(50),
SubGroup varchar(50),
Value int
)

insert into #mytable

(MainGroup, SubGroup, Value)

select 'Temporary Assignments','Internal','783' union all
select 'Temporary Assignments','Internal','225' union all
select 'Temporary Assignments','Internal','118' union all
select 'Temporary Assignments','Internal','1014' union all
select 'Operation''s Support','Internal','2006' union all
select 'Operation''s Support','Internal','6352' union all
select 'Operation''s Support','Internal','186' union all
select 'Operation''s Support','Internal','186' union all
select 'Technical','external','186' union all
select 'Technical','external','367' union all
select 'Technical','external','142' union all
select 'Technical','external','46' union all
select 'Loyalty','external','5600' union all
select 'Loyalty','external','194' union all
select 'Loyalty','external','3679' union all
select 'Loyalty','external','48' union all
select 'Loyalty','external','5520' union all
select 'Technical','external','60' union all
select 'Technical','external','140' union all
select 'Loyalty','external','1287' union all
select 'Technical','external','261' union all
select 'Loyalty','external','5533' ;
with cte as
(
select *,
ROW_NUMBER() over(PARTITION by MainGroup order by value desc) as 'rn'
from #mytable
)
select * from cte

--the results would look like this ...
Loyalty      external      1287      5
Loyalty      external      194      6
Loyalty      external      48      7
Operation's Support      Internal      186      4
Technical      external      140      5
Technical      external      60      6
Technical      external      46      7
Temporary Assignments      Internal      118      4