Beam forming IC

Here are the answers to your questions:

Q: What is the major advantage of summing up the 4 receive channels in Rx mode ? 

A: If the signals being summed are the same amplitude and in-phase with each other at the combiner, then there will be a 6 dB improvement in signal gain, relative to the same single channel but without a power combiner. 

Q: Can we treat it is a 4 separate channels ?   

A: The signals will always combine and become a single signal at the RF_IO output.  If they are not in-phase they will not combine coherently.

Q: Will it enhance the Rx gain ? 

A: Yes. See the answer to the first question.

Q: Is this useful for calculating the DOA ? 

A:  I don't know what "DOA" is.  Could you define this?