4 airlines to MAD
But the four airlines involved aren't offering exactly the same thing.
All offer cheap flights to Madrid, but IB Express also offers connections to many cities in S America and Africa (and a BA flight # in case you have loads of Avios.
In the long term, it is most important for BHX that the IB Express link thrives and increases frequencies, particularly as IAG will almost certainly increase useful connections through MAD (eg JNB restarting).
Spain is "on the way" to Africa and S America and journey times will be generally be shorter than via CDG or AMS, and even more so compared with DXB.