UPDATE tb1
SET c1=b.c1 ,
c2=b.c2
FROM b
WHERE tb1.c3 = b.c3 AND tb1.c4 = b.c4

insert into ...... on conflict(column_name) do ......

uxdb=# create table tbl_c (id int unique, name varchar(2));
CREATE TABLE
uxdb=# insert into tbl_c values (1, 'a');
INSERT 0 1
uxdb=# table tbl_c;
 id | name 
----+------
 1 | a
(1 row)
uxdb=# insert into tbl_c values (1, 'a');
ERROR: duplicate key value violates unique constraint "tbl_c_id_key"
DETAIL: Key (id)=(1) already exists.
uxdb=# insert into tbl_c values (1, 'a') on conflict(id) do update set name='b';
INSERT 0 1
uxdb=# table tbl_c;
 id | name 
----+------
 1 | b
(1 row)
 
uxdb=#