We have 5 or 6 copies of V4 and will not be upgrading at this time.
One reason is that old problems that should have been fixed, but did not get fixed, such as ripple edit should not be tolerated anymore.
Another is that network rendering was a bad choice assuming the choices were hardware, background, and network. Lets face it, for the most part Vegas is the choice of individual producers, not production houses and netwwork rendering at this time was not the wisest choice IMHO.
Jobs on Mandy.com and so forth are primarily for Avid and Final Cut (for the most part) and no amount of stomping or bitching is going to change that. I have never once seen a job postiong for "Vegas user wanted" in any forum. Premiere is gaining some ground, but Avid and Final Cut are the ones it if you are being honest and actually will listen.
Probably the best thing to do is to download the demo and try it out. It doesn't interfere with V4 but you can't open the V5 veg files in V4. You can have both on your system, though. In fact, I found that V5 didn't even change file associations. All veg files still tried to open in V4 regardless of which app created them.
Swarrine is right. At the moment Vegas is much better suited to individuals. And with the upgrade price at $200.00, I'd certainly upgrade your copy. But upgrading 5 seats? That's something else. As far as what you edit on, we do quite well at my shop with M100 systems and people rarely if ever ask us what we use to edit with. But all of our projects go from start to finish here-we don't farm anything out to other people so we don't need compatibility.