Well, because a 5 shader array is like a quad-core CPU. It's easier to fill a Dual-core than a quad-core or an octo-core.

So a 4 shader array will be fed easier with data than a 5 shader array. Thus, improving efficiency per array.

And, if you go with 4 shaders array, you have space for more arrays. Keeping the final shader number the same as previous GPUs, but with higher efficiency.

Some people mentioned something about the possibility of having 2 double precision capable shaders + 2 single precision shaders in such a 4 shader array. so a 2:2 ratio instead of the present 4:1. Would improve computational capabilities for sure, but nothing official from AMD.