A churn dash block is basically a 9 patch layout, so you need 3 patches across and 3 down. 4" finished is not easily divided by 3, as each patch needs to finish at 1.33333". A better way to go would be a 3" or 4.5" FINISHED block.
For a 3" finished, all the patches need to finish at 1", so cut your HST squares 2", then square the HSTs to 1.5", which will finish at 1". Cut the center 1.5" X 1.5", and cut the center side pieces 1" X 1.5"