I don't think AndyMark ever sold 4" hard plastic wheels. The 2009 6" wheels are out of stock
http://www.andymark.com/product-p/am-0284.htm. Based on them not appearing in a search of the site, my guess is that they've been discontinued. The 2010 8" hard plastic wheels are still available
http://www.andymark.com/product-p/am-0494.htm