VV4 will install in a separate folder from VV3. The SoFo website mentions that VV3 will not be disrupted by VV4. Let me know how yours goes first...just kidding. I have ACID 3 and ACID 4 running independently on the same PC.
Just be sure to remember the forward compatible NOT backward compatible rule with VV4. If you must open a VV3 created project in VV4 be sure to save it in VV4 with a new name!
Steve