Untitled

Select id where all user id match
+ - - - - - - - - - - - + - - - - - +
¦ fk_conversation       ¦   fk_user ¦
+ - - - - - - - - - - - + - - - - - +
¦ 1                     ¦     2     ¦
+ - - - - - - - - - - - + - - - - - +
¦ 1                     ¦     3     ¦
+ - - - - - - - - - - - + - - - - - +
¦ 1                     ¦     4     ¦
+ - - - - - - - - - - - + - - - - - +
¦ 2                     ¦     2     ¦
+ - - - - - - - - - - - + - - - - - +
¦ 2                     ¦     4     ¦
+ - - - - - - - - - - - + - - - - - +

select fk_conversation
from my_table
where fk_user in ( ARRAY VALUES )
and fk_conversation in (
    select fk_conversation
    from my_table
    group by fk_conversation having count(distinct fk_user) = ARRAY SIZE
)
group by fk_conversation
having count(distinct fk_user) = ARRAY SIZE;

select fk_conversation
from table
where fk_user in ( comma separated array_values )
group by fk_conversation
having count(fk_conversation) = array_size