ESRI does not support the 4.0 framework until 10.1. Even then, they do not require it nor do they check whether or not it is installed.
If you search the forums you can find various posts where people have used the 4.0 framework in earlier versions of ESRI software even though it is not supported.
In version 10, ESRI had a goal of support multiple installs on the same machine but then abandoned that effort leaving only additional complexity. So, Rich is correct in that your only option here is to use virtual machines.