I have a few-years-old grass green Lacoste polo shirt (size 7) in my closet I've been meaning to sell. I pulled it out today, and now I want one that fits. There is some confusion on Ebay about the sizing scheme, specifically whether a 4 or a 5 is a medium size. I wear a 40 chest or usually a medium shirt size. I want it slim but not tight. What size do I need?