I thought it was only 2GB ?
2GB is the memory limit for software in XP2, unless the 3GB switch is used. With 4GB of memory, XP2 will utilize the other 2GB (or most of it) for the operating system (or 1GB if the 3GB switch is used), leaving the other half for user software. So it's still somewhat useful to have 4GB even without the 3GB switch, because you get the full 2GB for software (no Windows services eating it up).
But there's another issue. I remember reading all the 3GB switch -related topics on this forum a while back, and what I found out was that an application has to be Large Address Aware (LAA) to be able to use over 2GB of memory. Samplitude is not. Therefore, the 3GB switch doesn't really do much good with Samplitude unless you are using other applications at the same time. All VSTis, sample libraries and such you use in Samplitude are using Samplitude's memory space (which is capped at 2GB), so the 3GB switch does not help at all with VSTis and sample libraries either.
This was the case just a few weeks ago (with v9.1.1), if something has changed after that, please correct me. I cannot read the German forum, but the English link posted by Sebastian doesn't really prove otherwise.