I recently read a letter from the S&W Historical Foundation stating only 150 model 4536 were produced. If so, this would be one of the most rare of S&W 3rd Gen models. Is this true?
Not true, while production letters are one of the best sources of information on how many units were made, they are not infallible.
The production letter you speak of concerns serial numbers in the THA7xxx range, but that's not all of them. I have a production letter copy for TEP4025 stating 736 units produced and I've also observed TFE6873.