You are right that there are supposed to have been four witnesses who saw Adams around the time of the crime.  Mrs. Salmon, of course, saw him from her window.  Henry MacDougall nearly ran him over while driving home.  Mr. Wheeler lived next door to Mrs. Parker and also saw Adams through a window just as Mrs. Salmon did.  The fourth witness is mentioned in the last sentence of the paragraph in which all these other witnesses are listed.  The fourth witness is not given a name.  It is simply stated that this witness (we do not even know whether it was a man or a woman) saw Adams “in Laurel Avenue.”

So, there is a fourth witness but we know nothing about that person.