with a as 
(select 'A1' as name, 1 as dt from dual
 union all
 select 'A2' as name, 3 as dt from dual
  union all
 select 'A3' as name, 3 as dt from dual
   union all
 select 'A4' as name, 4 as dt from dual
   ),
 b as 
 (select 'B1' as name, 1 as dt from dual
 union all
 select 'B2' as name, 1 as dt from dual
 union all 
 select 'B3' as name, 3 as dt from dual
  union all
 select 'B4' as name, 3 as dt from dual)
, y as
(select min(name) name2, dt from b group by dt)
select a.name, a.dt,
 case when
   coalesce(lag(a.dt) over (order by a.dt, a.name), -1) != a.dt
 then y.name2 end
from a left join y on a.dt=y.dt
order by a.name, a.dt

select a.name, a.dt,
 case when
    coalesce(lag(a.dt) over (order by a.dt, a.name), -1) != a.dt
 then 
    (select min(name) from b where b.dt between a.dt and a.dt + 3)
 end as b_name
from a
order by a.name, a.dt

, d as
(
 select dt from a group by dt
)
, y (name2, dt) as
(
 select min(b.name) name2, d.dt
 from b join d on b.dt between d.dt and d.dt + 3
 group by d.dt
)
select a.name, a.dt,
 case when
   coalesce(lag(a.dt) over (order by a.dt, a.name), -1) != a.dt
 then y.name2 end as b_name
from a left join y on a.dt=y.dt
order by a.name, a.dt

select name1 as name, dt, min(case when rn = 1 then name2 end) b_name
from
(
       select a.name as name1, b.name as name2, a.dt,
              row_number() over (partition by a.dt order by a.name, b.name ) rn     
       from a left join b on b.dt between a.dt and a.dt + 3
)
group by name1, dt