In MIMO, the maximum possible rank of the channel, given by 4, is equal to 4 Layers.
Does this may result in a decoding failure under some channel conditions?
Yes, when multipaths are somehow correlated.
Is there more information for this please?
Check the precoding matrix and do the correlation of beamforming weights.
Ok. I know the precoding matrix how to get it, but correlation of weight - how should I know it?