SQL> ed
Wrote file afiedt.buf

  1  with t(client_id,dat) as (
  2  select 1,date '2020-01-20' from dual union all
  3  select 1,date '2020-01-31' from dual union all
  4  select 1,date '2020-02-01' from dual union all
  5  --
  6  select 2,date '2020-01-20' from dual union all
  7  select 2,date '2020-01-23' from dual union all
  8  select 3,date '2020-01-30' from dual union all
  9  --
 10  select 3,date '2020-01-20' from dual
 11  )
 12  ,tt as (
 13  select t.*
 14  ,lead(dat) over(partition by client_id,trunc(dat,'mm') order by dat) l --to_char(dat,'yyyymm')
 15  from t)
 16* select unique client_id from tt where l-dat<4
SQL> /

 CLIENT_ID
----------
         2

SQL> ed
Wrote file afiedt.buf

  1  with t(client_id,dat) as (
  2  select 1,date '2020-01-20' from dual union all
  3  select 1,date '2020-01-20' from dual union all
  4  select 1,date '2020-01-31' from dual union all
  5  select 1,date '2020-02-01' from dual union all
  6  --
  7  select 2,date '2020-01-20' from dual union all
  8  select 2,date '2020-01-23' from dual union all
  9  select 2,date '2020-01-30' from dual union all
 10  --
 11  select 3,date '2020-01-20' from dual
 12  )
 13  ,tt as (
 14  select t.*
 15  ,lead(trunc(dat)) over(partition by client_id,trunc(dat,'mm') order by dat) l
 16  from t)
 17* select unique client_id from tt where l-trunc(dat)<4 and l-trunc(dat)>=1
SQL> /

 CLIENT_ID
----------
         2