Not if you upgraded (you can run the programs at the same time however, no computer problems). When you upgrade V5 "overwrights" V4, speaking legaly. So, you can't even sell an old copy if you upgraded it.
Your Vegas license would cover both 4 and 5 even if you upgraded.
Since you can run multiple instances of Vegas at once, you can legally run a instance of Vegas 4 and an instance of Vegas 5 at the same time on the same computer.
The Vegas license also allows you to use Vegas on another machine provided that you never run Vegas on both computers at the same time
The license that covers the instance is a single license. So if you upgraded from 4 to 5, you cannot sell your copy of Vegas 4 and still use Vegas 5. But if you wanted to run both Vegas 4 and Vegas 5 on the same computer you can legally do so.
That's what I would've thought, too. But according to this post, Sony's customer services says that not only can you use Vegas 4 and Vegas 5 on the same computer at the same time, not only can you run Vegas on more than one machine (as you say, so long as it's not at the same time), but also you CAN sell your copy of Vegas 4 after upgrading to Vegas 5. The person you sell it to, though, is not elligible to upgrade as well.
According to that particular CS rep, anyhow. I'd think this is all "clearly stated" somewhere, but I haven't found it yet.
You can indeed use both versions of the Vegas, legally and operationally. I checked with Sony Vegas CS, and was told the same thing as the above referenced thread stated. I have since sold my copy of Vegas4. All you have to do is call with the info, and they will generate a new number for the software. The only caveat is the copy of Vegas4 is NOT eligible for any upgrades. But in answer to the original posted question, both work just fine together on the same machine.
Wow. They must if changed their tune when Sony bought them. Last I rememeber SF saying is that you can't sell/use the version that was upgraded. Woohoo! :)