It's part of a Hoya 49mm Close-up kit. +4 refers, I believe, to diopters. It's just like a filter that screws to the lens. Here's a link to a 58mm set http://stores.tomshardware.com/search_getprod.php/masterid=728780// My original question in this thread was to wonder if I could get something similar for my Leitz 100/2.8. But apparently 60mm is a very unusual size, so I ended up getting the Elpro 1:2~1:1 for that lens.