Pinwheels are made from 4 - HSTs. For a finished 4" pinwheel then each corner of the HST should
finish at 2".
When doing HSTs, I use method 2 in the following tutorial
http://www.thatgirlthatquilt.com/201...es-3-ways.html
The rule of thumb for assembling HSTs this way is "
finished size plus 7/8". So a 3" square will be adequate using method 2 - giving you a little wiggle room for trimming the final pinwheel into a perfect square.
My only concern about Jenny Donan's method (method 1 in the first link) is that the resulting edges are not on the straight of the goods and can stretch, If you do want to do that method, the following site even has a chart for measurements
http://whipup.net/2011/03/17/guest-b...are-triangles/