Yes, the extension you require to extend the WS2 to a WS4 is the EXT2, which is 120mm in length. I've listed the details below.
Description: Extension 2 (120mm)
Order Code: 010702
Web link: www.rycote.com/products/010702
If you require the larger Windjammer to suit the WS4, the details are below also.
Description: Windjammer WJ 4
Order Code: 021504
Web link: www.rycote.com/products/021504