The part number sounds about right mate, but they don't come as standard on the ST4, cos the ST4 cycle parts are the same as the ST2. The Ohlins only come on the 4S.
I'm not sure you'll get a secondhand one, partly because there aren't that many 4S's being broken for spares and secondly because secondhand ones might be worn and need a rebuild anyway - in which case it'll cost you near on the same as a new one.
There is a likely looking suspect
Here on eBay. It's listed as a Monster part but since the swing arms on a 900 Monster and an ST are pretty much identical it may well fit. Can't guarantee it, you'd need someone who knows what they're talking about for that.