I am also in total support of remaining open to other differential aetiologies. For example, I have attended to one case where piriformis shortening & irritation was secondary to excessive supination of the hindfoot. I have also heard of a few cases where weakened gluteals present a very important aetiology.

But I will not be quick to dismiss Ginger's postulation that L4 is important in Piriformis Synd; it sounds quite logical.