Create index idx_1 on example_table(A) ;
Create index idx_2 on example_table(A,B,C,D);

Maybe.

While both are candidates for searches on column A, you may find that the single-column index is used in some cases where the multicolumn index isn't.

For example, let's create a table and create a three-column index on it:

create table t ( 
  c1 constraint pk primary key, c2 not null, c3 not null, c4 not null
) as 
  select level c1, ceil ( level / 100 ) c2, 
         date'2021-01-01' + mod ( level, 37 ) c3,
         lpad ( 'stuff', 100, 'f' ) c4
  from   dual
  connect by level <= 10000;
  
create index i231 on t ( c2, c3, c1 );

A query on the index's leading column returns 1% of the rows, but the optimizer chooses a full table scan:

set serveroutput off

select * from t 
where  c2 = 1;

select * 
from   table(dbms_xplan.display_cursor(null, null, 'BASIC LAST'));

----------------------------------    
| Id  | Operation         | Name |    
----------------------------------    
|   0 | SELECT STATEMENT  |      |    
|   1 |  TABLE ACCESS FULL| T    |    
----------------------------------

But create an index on this column alone and the optimizer does use it:

create index i2 on t ( c2 );

select * from t
where  c2 = 1;

select * 
from   table(dbms_xplan.display_cursor(null, null, 'BASIC LAST'));

----------------------------------------------------    
| Id  | Operation                           | Name |    
----------------------------------------------------    
|   0 | SELECT STATEMENT                    |      |    
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| T    |    
|   2 |   INDEX RANGE SCAN                  | I2   |    
---------------------------------------------------- 

So... why is this?

There are a couple of reasons.

First up, the single-column is smaller - there's less data in it. So it's faster to read it.

Secondly the multi-column index has a much higher clustering factor - over 20x higher:

select index_name, leaf_blocks, clustering_factor
from   user_indexes
where  table_name = 'T';

INDEX_NAME   LEAF_BLOCKS   CLUSTERING_FACTOR   
PK                    20                 170 
I231                  37                7250 
I2                    20                 170 

The clustering factor is a key determinant of how effective an index is and thus how likely the optimizer is to choose it (lower = better). The fewer columns there are, the lower the clustering factor is likely to be.

It's a good idea to create as few indexes as you can. But as this example shows there may be cases where creating "redundant" indexes leads to faster queries.