SQL & PL/SQL :: Assign Doc_num From Doc_id Table To 4 Customers In Rent Table Randomly

I've 2 table with below colums

Create table rent (customer_id number(10),
doc_num varchar2(20)
);
Create table doc_id (doc_num varchar2(20));

Insert into rent(customer_id) values (1);
Insert into rent(customer_id) values (2);
Insert into rent(customer_id) values (3);
Insert into rent(customer_id) values (4);
[code]...

Now my requirement is i need to assign doc_num from doc_id table to 4 customers in rent table randomly. I mean update doc_num in rent table from doc_id table randomly. how to write update statement.