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#11
Both the bios and Windows are seeing the full 6 GB, but for whatever reason the bios is keeping the 4 GB back as hardware reserved. Had a look at the various manuals for your laptop but there wasn't a whole lot of info regarding what bios settings are available. If it is there, have you tried "load failsafe deafults" (or however it may be worded in yours)? Seeing as it won't boot with just the 4 GB stick in, would have to try this with both it and the 2 GB installed.
That it won't boot at all with just the 4 GB stick in would lead me to believe that it is possibly a bad stick, or that it is just incompatible with the computer in general.