Statistically speaking you want the first quartile, so borrowing on Matt's solution I would just shift a few things to make it intuitive ...

with MYcte as (

selectMAINGROUP,vALUE, NTILE(4) OVER (PARTITION BY MAINGROUP ORDER BY VALUE ASC) AS NTILE_GROUP

FROM#MYtABLE

)

select MAINGROUP, VALUE, NTILE_GROUP AS FIRST_QUARTILE

FROM MYcte WHERE NTILE_GROUP=1

Here in your code imples you want the first quartile, the bottom 25%. That keeps confusion from setting in when you read your code in four months 😎