create table bookmarks(bookmark_id number,user_id number);

insert into bookmarks(bookmark_id,user_id) values(1,100);
insert into bookmarks(bookmark_id,user_id) values(2,100);
insert into bookmarks(bookmark_id,user_id) values(3,100);
insert into bookmarks(bookmark_id,user_id) values(4,101);
insert into bookmarks(bookmark_id,user_id) values(5,102);
insert into bookmarks(bookmark_id,user_id) values(6,103);
insert into bookmarks(bookmark_id,user_id) values(7,103);
insert into bookmarks(bookmark_id,user_id) values(8,103);
insert into bookmarks(bookmark_id,user_id) values(9,103);
insert into bookmarks(bookmark_id,user_id) values(10,103);

commit;

SQL> select * from bookmarks;

BOOKMARK_ID    USER_ID
----------- ----------
          1        100
          2        100
          3        100
          4        101
          5        102
          6        103
          7        103
          8        103
          9        103
         10        103

10 rows selected.

SQL> select user_id
  2  from bookmarks
  3  group by user_id
  4  having count(*)<5;

   USER_ID
----------
       100
       102
       101