The Version 4.0 for the 7100 isn't officially available anywhere by any carrier yet.

However, if you managed to get a hold of a copy of it, make sure you install BlackBerry Desktop 4.0 first. Then run the 7100 installer. Then delete the VENDOR.XML file. Then run BlackBerry Desktop and follow instructions.

I wouldn't recommend upgrading a 7100 just yet, though. Wait a little bit. The 7100 is special in that it already has many 4.0 features already.