Why are you still running VMware Fusion 3.0.1, you should have upgraded to the latest VMware Fusion 3.x which is VMware Fusion 3.1.3.
Unless things have changed with VMware Fusion 4, and I don't think they have, you can only have one version of VMware Fusion installed at a time however you can revert to a previous version if you don't like VMware Fusion 4 after trying the Fully Functional Free 30 day Trial.
Also you can/should backup the Virtual Machines before upgrading or at the minimum take a Snapshot so you have an easy avenue to revet to a known good working state if needing to revert back to a previous version of VMware Fusion.
Have a look at: Downgrading from Fusion 4 to Fusion3 (2005310)
