My guess is they knew #4 wasn't in Durham that day.
So, she identifies #4, who they know was out of town; did bells and whistles not go off right that second?
I mean, if that is true, all 4 boys have alibis. (According to Mort, Finnerty does, too.)
It certainly appears like the investigator was not anxious to follow up that ID.
That's why the line-up protocol I read said the person presenting the pictures to the accuser should be someone unfamiliar with the suspects.
Instead, it was Gottlieb in charge of the investigation. At least he got her to clarify her "only without the mustache" comment by asking, "OK, so the person had a mustache?".
Crystal's "Yes" probably saved poor Evans.