As much as the style guidelines recommend "No crystal malts" in a vienna, I think a well made vienna can contain a small amount. In fact, there have been some NHC gold medal winners who have added substantial amounts of crystal malts to their award winning Viennas. So, I don't think 4 oz is out of the question here either. I would think you could either use Caramunich II (37-40L) or Caramunich III (57-60L) for this depending on the color you were looking for in your final product and what type of crystal sweetness you desired. III is bit more intense than II, but not by much IMO, especially when only 4 oz are to be added.