Rank 1 means 1 stream of data.
Network and device can still be using 2 or 4 antennas (for spatial diversity).
As UE RF condition improves, network can start sending separate stream for each of antenna receiver (if 2 antenna receiver Rank 2 will make 2 separate streams of data, thereby doubling throughput).
Concept is same across technologies. Mechanism can be different.
I personally do not prefer word ‘layer’. Not sure but don’t think its a standard term.
When you get to MU-MIMO with CA, word 'layer gets a whole lot confusing.
So for purpose of this, yes stream is a layer.
I have slight disagreement on layer and stream definition. As an example , for rank 1 case - there can only be 1 layer but 1,2 or 4 streams. These 1,2 or 4 streams are in case of transmit diversity with 1,2 or 4 antenna support. Layers and streams will map 1 to 1 in case of spatial multiplexing,ex 2 layers = 2 streams.