I asked our Fb Product Manager about this, and he said you would need to do this in the other direction: if you order FB4 now, you can get a free upgrade to FB 4.5 when it's available by calling Customer Service.
If you speak with Customer Service directly, they should be able to straighten things out.
Thanks for your reply.
Problem with the solution you suggested is that I can not find another
version of Flash Builder than 4.5.
At http://www.adobe.com/products/ there is just "Flash Builder™ 4.5" nothing
But I guess contacting Customer Service, as soon as I get back from my
holiday trip, might clarify how to get the mysterious FB 4.