Well, im Just answering this by myself because i want to help others.
In efect you cant use 4-wire TX as a Slave if yoou dont keep the i2s RX slave Instance working. I did several tests and this is how it works.
For mbed I2S lirbrary you have to:
Create an I2S Receiver object and configure it,
I2S* i2s_rx = new I2S( I2S_RECEIVE, p17, p16, p15 );///this can be static if you need to and, belong to a class
//int rxData = i2s_rx.read();//for testing
i2s_rx.stop();//this function wont deactivate the IRQ for TX/RXFIFO this must be a BUG
Only after this element you can create an I2S transmitter element as follows:
I2S* i2s_tx(I2S_TRANSMIT, p11, true);//this can be static if you need to, and belong to a class
int yes = i2s_tx.setup_ok();
i2s_tx.stop();//this function wont deactivate the IRQ for TX/RXFIFO this must be a BUG
Regards, in contrary case you can use 4wire RX but before you need 3 wire master TX