I had version 3 of Lightroom installed on an old computer and then upgraded to version 4 on a new computer. I asked a similar question of Adobe and this is the response I got.
I am glad that we've resolved the Installation issue by informing you that you will not have to install Lightroom 3 in order for
you to install the upgrade version which is Lightroom 4.0
However while installing Lightroom 4 on a new computer you will be asked to enter the previous version serial number.
I think this is somewhat similar to what you are doing.