It even might be that they found out that for 4D it must be enough shaders to be more effective than 5D. It depends on trig vs other calculations. It very well might be that from 1920 shaders in 4D they can extract much more performance than in 5D but in ~1200sp count it could be vice versa. There is so much variables in it. Think also about the fact that 4D change will improve DP performance quite a bit, it was in 5D only those 4sp's not the fat one which was used in DP calc.