Unfortunately, we do not have any mechanism in place currently to do this. Is there a reason that you cannot work solely in 4.0?
Not all our customers have software maintenence purchased so we need to sell them upgrade and update application to v4.0 afterwards. That's extra effort.
Even if we could upgrade everyone then in transition phase there will always be customers both using v2.2 and v4.0.
We need to support both versions for some time and that requires two developer setups.
Now with RhoElements v4.1 comming we might need 3rd setup for testing and developing the new version.
It would be usefull to have all versions in one RhoStudio and pick which version to use just in build.yml or so.
You may be able to achieve this through the use of gemsets via RVM. However, if you are not using Mac OS this will not work since RVM does not work with Windows. If you are using Mac OS for development, you can look here, RVM: Ruby Version Manager - RVM Gemsets to learn about gemsets. You can create a new gemset for each version you need to maintain and then switch the gemset when you need to work on a different version.