Well it's not really that is HAS more than we have now. Technically because we start over with the plugins it has less.What does 4.0 have that we don't already have? I'm of course looking at it from a moderator perspective AND user perspective
But since it's been out for about a year, We need to at least consider the upgrade. It's going to be increasingly more difficult to continue supporting something that isn't supported anymore. Like when i get requests - "the classifieds need to be more useful" - its not like i can just go find a new classifies plugin anymore, they are all being updated and maintained for the new version.
We can leave it alone for now, but, while it is a big undertaking, 4.0 is re written and finally follows correct web standard and as new security flaws get fixed, 4.0 will get the fixes, 3.84 will not.
I would never suggest an upgrade right away, but since it's been out in the wild for a bit, has had 4 sub releases (4.0.4) I think its worth the time to see how painful it is to actually do the upgrade.