Внутреннее соединение, только если поле не пустое

I could have done what I want with two request, but I want to make it with only one.

In fact, I have 4 tables with :

Table 1 : id, sub-id
Table 2 : id, sub-id
Table 3 : id, login
Table 4 : id, login

I make a request like that :

SELECT Table1.id, Table1.sub-id, Table2.id, Table2.sub-id, 
   Table3.login, Table4.login FROM Table1
   INNER JOIN Table2 ON (Table1.id = Table2.id AND Table1.sub-id = Table2.sub-id)
   INNER JOIN Table3 ON (Table3.id = Table1.id)
   INNER JOIN Table4 ON (Table4.id = Table1.id)
   WHERE Table1.id = "my_id" AND Table1.sub-id = "my_subid"

I want to join Table3 only if id is not empty, if it is empty, I join Table4.

Do you have any ideas please ?

I've heard that a left join could help, but I'm not that accustomed to these keywords so ...?