Concatenate Multiple columns

I would like to concatenate contact1 & 2 and contact 3 & 4. I would like 
to show only distinct contacts for each person_id. However, there can be 
duplicate contacts for different person_ids.
Any feedback would be greatly appreciated. Please note that the 
performance of the query is very important here. I'm looking for an Oracle SQL code. I'm working on Toad.
Thank you!

create table emp_contact(

person_id numeric,

contact1 varchar(50),

contact2 varchar(50),

contact3 varchar(50),

contact4 varchar(50)

);

insert into emp_contact values (1,'jill', 'jill, mary', 'amy, jack', 'david, abby, mike');

insert into emp_contact values (2,null, 'jill, mary', 'amy, jack', null);

insert into emp_contact values (3,null, null, null, null);

insert into emp_contact values (4,'mike', 'jill, mary, abby', 'amy, jack, jill', null);

Output: