One of the eovia guys clarified this on the yahoo Carrara group. The $189 is the electronic download that will be available when it's released. Here's the post:

The price for the Electronic Download (ESD) version of the Carrara 4
Pro upgrade will be $189 when available. The Physical version of
the upgrade is $199.

Only the Physical version is available for pre-book. The ESD for
$189 will become available once the product starts shipping.

Best-
Bob Stockwell
Eovia Corp.

Regards,
Eric Winemiller
Digital Carvers Guild
3D plug-ins for Carrara
http://digitalcarversguild.com

Eric Winemiller
Digital Carvers Guild
Carrara and LightWave plug-ins